100 MySQL Practice Questions & Answers⌗

A comprehensive, production-grade collection of 100 real-world SQL interview questions and battle-tested solutions built directly on the official MySQL employees relational database.

[!tip] 🚀 Practice Online in Your Browser (No Setup / Installation Required) Practicing on a work/restricted laptop without MySQL installed? You can run and practice all these queries live with the preloaded dataset right in your browser: 👉 Open the Web SQL Playground (Runs 100% in-browser, preloaded with Employees & ClassicModels data).

[!info] Quick Navigation

Part 1: Basic SELECT, Filtering & Sorting (Q1 – Q15)

Part 2: String & Date Functions (Q16 – Q30)

Part 3: Aggregations, GROUP BY & HAVING (Q31 – Q45)

Part 4: Relational Joins Mastery (Q46 – Q60)

Part 5: Subqueries, CTEs & Set Operations (Q61 – Q75)

Part 6: Window Functions & Analytical SQL (Q76 – Q90)

Part 7: Advanced Business Logic & Edge Cases (Q91 – Q100)

🛠️ Schema Relationship Cheat Sheet⌗

  employees (emp_no)
      │
      ├──< dept_emp (emp_no, dept_no) >── departments (dept_no)
      │
      ├──< dept_manager (emp_no, dept_no) >── departments (dept_no)
      │
      ├──< salaries (emp_no, salary, from_date, to_date)
      │
      └──< titles (emp_no, title, from_date, to_date)

[!tip] Active Records Rule in Employees Dataset Active department assignments, titles, and salaries are marked by to_date = '9999-01-01'.

🟢 Part 1: Basic SELECT, Filtering & Sorting⌗

Focus: Fundamental filtering, comparison operators, and projection.

Q1. Employees Hired After 1990⌗

[!question] Problem Statement Select all columns for the first 10 employees hired after January 1, 1990, ordered by hire date ascending.

[!tip] Concepts Tested WHERE, Date Comparison, ORDER BY, LIMIT

[!success]- Reveal SQL Solution
SELECT * 
FROM employees 
WHERE hire_date > '1990-01-01' 
ORDER BY hire_date ASC 
LIMIT 10;
Explanation: Filters rows using WHERE hire_date > '1990-01-01', sorts the dataset chronologically with ORDER BY hire_date ASC, and limits the output to the first 10 records.

Q2. All Distinct Historical Job Titles⌗

[!question] Problem Statement Retrieve all unique job titles ever held by employees across the entire organization.

[!tip] Concepts Tested SELECT DISTINCT, Column Projection

[!success]- Reveal SQL Solution
SELECT DISTINCT title 
FROM titles 
ORDER BY title ASC;
Explanation: Uses DISTINCT to eliminate duplicate title entries across historical employment records, returning each distinct job position.

Q3. Pattern Matching on First and Last Names⌗

[!question] Problem Statement Find all employees whose first name starts with ‘B’ and whose last name ends with ’s’.

[!tip] Concepts Tested LIKE Wildcard Matching (%), Logical AND

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, gender, hire_date 
FROM employees 
WHERE first_name LIKE 'B%' 
  AND last_name LIKE '%s';
Explanation: 'B%' matches any string beginning with uppercase ‘B’, while '%s' matches any string ending in ’s’.

Q4. Female Employees Hired in Late 80s⌗

[!question] Problem Statement List all female employees (gender = 'F') hired between January 1, 1985 and December 31, 1989.

[!tip] Concepts Tested BETWEEN ... AND ..., Equality Filter

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, gender, hire_date 
FROM employees 
WHERE gender = 'F' 
  AND hire_date BETWEEN '1985-01-01' AND '1989-12-31' 
ORDER BY hire_date ASC;
Explanation: BETWEEN is inclusive on both boundaries (1985-01-01 and 1989-12-31).

Q5. Substring Search in First Names⌗

[!question] Problem Statement Find all employees whose first_name contains the substring ‘ar’ anywhere within the name.

[!tip] Concepts Tested LIKE '%ar%' (Case-Insensitive in default collation)

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name 
FROM employees 
WHERE first_name LIKE '%ar%' 
ORDER BY first_name ASC;
Explanation: The surrounding % wildcards allow any characters before and after ‘ar’.

Q6. Top 5 Highest Salaries Ever Recorded⌗

[!question] Problem Statement Retrieve the top 5 highest distinct salary values recorded in company history.

[!tip] Concepts Tested DISTINCT, ORDER BY DESC, LIMIT

[!success]- Reveal SQL Solution
SELECT DISTINCT salary 
FROM salaries 
ORDER BY salary DESC 
LIMIT 5;
Explanation: Orders the salaries table in descending order and returns the 5 highest historical compensation levels.

Q7. Long Department Names⌗

[!question] Problem Statement Find all departments that have a dept_name longer than 15 characters.

[!tip] Concepts Tested CHAR_LENGTH() vs LENGTH()

[!success]- Reveal SQL Solution
SELECT dept_no, dept_name, CHAR_LENGTH(dept_name) AS name_length 
FROM departments 
WHERE CHAR_LENGTH(dept_name) > 15;
Explanation: CHAR_LENGTH() counts characters rather than bytes, making it safe for multi-byte Unicode strings.

Q8. Active Employees Outside Department d005⌗

[!question] Problem Statement Select all currently assigned employees who do not belong to department ‘d005’.

[!tip] Concepts Tested != or <>, Active Record Filtering (to_date = '9999-01-01')

[!success]- Reveal SQL Solution
SELECT emp_no, dept_no, from_date, to_date 
FROM dept_emp 
WHERE dept_no <> 'd005' 
  AND to_date = '9999-01-01';
Explanation: In the MySQL employees sample database, active status is represented by to_date = '9999-01-01'. <> denotes inequality.

Q9. Current Salaries in 60k to 70k Range⌗

[!question] Problem Statement Retrieve all active salary records where the compensation is between $60,000 and $70,000 inclusive.

[!tip] Concepts Tested BETWEEN, Active Record Filter

[!success]- Reveal SQL Solution
SELECT emp_no, salary, from_date, to_date 
FROM salaries 
WHERE salary BETWEEN 60000 AND 70000 
  AND to_date = '9999-01-01' 
ORDER BY salary DESC;
Explanation: Combines active date filtering (to_date = '9999-01-01') with an inclusive numeric range check.

Q10. Employees Born in February⌗

[!question] Problem Statement Find all employees who were born in the month of February, regardless of year.

[!tip] Concepts Tested MONTH() or EXTRACT(MONTH FROM ...)

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, birth_date 
FROM employees 
WHERE MONTH(birth_date) = 2;
Explanation: MONTH(birth_date) extracts the 1-indexed calendar month (1=Jan, 2=Feb).

Q11. Engineering and Management Titles⌗

[!question] Problem Statement List all unique job titles that include the words ‘Engineer’ or ‘Manager’.

[!tip] Concepts Tested LIKE, Logical OR

[!success]- Reveal SQL Solution
SELECT DISTINCT title 
FROM titles 
WHERE title LIKE '%Engineer%' 
   OR title LIKE '%Manager%' 
ORDER BY title ASC;
Explanation: Searches title variations like ‘Senior Engineer’, ‘Assistant Engineer’, and ‘Manager’.

Q12. Round Number Salaries⌗

[!question] Problem Statement Find all distinct historical salary amounts that are exact multiples of 1,000 (e.g., 65000, 72000).

[!tip] Concepts Tested Modulo Operator MOD() or %

[!success]- Reveal SQL Solution
SELECT DISTINCT salary 
FROM salaries 
WHERE salary % 1000 = 0 
ORDER BY salary DESC;
Explanation: salary % 1000 = 0 checks that the salary has zero remainder when divided by 1,000.

Q13. The 10 Oldest Employees⌗

[!question] Problem Statement Retrieve the 10 oldest employees in the database based on their birth date.

[!tip] Concepts Tested ORDER BY birth_date ASC, LIMIT

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, birth_date, hire_date 
FROM employees 
ORDER BY birth_date ASC 
LIMIT 10;
Explanation: Ascending sort on birth date places earlier dates (older individuals) at the top.

Q14. Currently Active Department Managers⌗

[!question] Problem Statement Find all department managers who are currently active.

[!tip] Concepts Tested Equality check on open-ended date literal

[!success]- Reveal SQL Solution
SELECT emp_no, dept_no, from_date, to_date 
FROM dept_manager 
WHERE to_date = '9999-01-01';
Explanation: In this schema, dept_manager records with to_date = '9999-01-01' signify active assignments.

Q15. Equal Length First and Last Names⌗

[!question] Problem Statement List all employees whose first name and last name have the exact same character count.

[!tip] Concepts Tested CHAR_LENGTH(), Column-to-Column Comparison

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, CHAR_LENGTH(first_name) AS name_len 
FROM employees 
WHERE CHAR_LENGTH(first_name) = CHAR_LENGTH(last_name) 
LIMIT 20;
Explanation: Compares the character lengths of two distinct columns on each row.

🟢 Part 2: String & Date Functions⌗

Focus: String manipulation, date math, age calculations, and temporal patterns.

Q16. Formatted Full Name (LASTNAME, Firstname)⌗

[!question] Problem Statement Display each employee’s full name in the format: ‘LASTNAME, Firstname’ (e.g. ‘SMITH, John’).

[!tip] Concepts Tested CONCAT(), UPPER()

[!success]- Reveal SQL Solution
SELECT emp_no, 
       CONCAT(UPPER(last_name), ', ', first_name) AS formal_name 
FROM employees 
LIMIT 15;
Explanation: Converts last_name to uppercase and concatenates it with a comma, space, and the capitalized first name.

Q17. Current Employee Age⌗

[!question] Problem Statement Calculate the current age in complete years for every employee as of today.

[!tip] Concepts Tested TIMESTAMPDIFF(), CURDATE()

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, birth_date, 
       TIMESTAMPDIFF(YEAR, birth_date, CURDATE()) AS current_age 
FROM employees 
LIMIT 15;
Explanation: TIMESTAMPDIFF(YEAR, d1, d2) accurately calculates whole elapsed calendar years between two dates without leap-year rounding errors.

Q18. Hire Age in Years⌗

[!question] Problem Statement Calculate how old each employee was when they were hired.

[!tip] Concepts Tested TIMESTAMPDIFF() with two date columns

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, birth_date, hire_date, 
       TIMESTAMPDIFF(YEAR, birth_date, hire_date) AS age_at_hire 
FROM employees 
LIMIT 15;
Explanation: Calculates elapsed years between birth_date and hire_date.

Q19. Employees Hired on Mondays⌗

[!question] Problem Statement Find all employees who were hired on a Monday.

[!tip] Concepts Tested DAYNAME() or WEEKDAY()

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, hire_date, 
       DAYNAME(hire_date) AS day_of_week 
FROM employees 
WHERE DAYNAME(hire_date) = 'Monday' 
LIMIT 15;
Explanation: DAYNAME(hire_date) returns the full English name of the day (e.g., ‘Monday’).

Q20. Custom Date Formatting⌗

[!question] Problem Statement Format each employee’s hire date as ‘Mon DD, YYYY’ (e.g., ‘Jan 15, 1990’).

[!tip] Concepts Tested DATE_FORMAT() specifiers (%b, %d, %Y)

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, 
       DATE_FORMAT(hire_date, '%b %d, %Y') AS formatted_hire_date 
FROM employees 
LIMIT 15;
Explanation: %b gives 3-letter month abbreviation, %d gives 2-digit day, and %Y gives 4-digit year.

Q21. Employees with Over 30 Years of Service⌗

[!question] Problem Statement Find all employees whose tenure exceeds 30 years from their hire date.

[!tip] Concepts Tested DATE_SUB() or INTERVAL Date Arithmetic

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, hire_date 
FROM employees 
WHERE hire_date <= DATE_SUB(CURDATE(), INTERVAL 30 YEAR) 
ORDER BY hire_date ASC 
LIMIT 15;
Explanation: DATE_SUB(CURDATE(), INTERVAL 30 YEAR) calculates the cutoff date 30 years prior to today.

Q22. Generate Corporate Email Addresses⌗

[!question] Problem Statement Generate a standardized corporate email address in all lowercase: firstname.lastname@company.com.

[!tip] Concepts Tested LOWER(), CONCAT()

[!success]- Reveal SQL Solution
SELECT emp_no, 
       LOWER(CONCAT(first_name, '.', last_name, '@company.com')) AS corporate_email 
FROM employees 
LIMIT 15;
Explanation: LOWER() converts all characters to lowercase after string concatenation.

Q23. First Names with Odd Character Counts⌗

[!question] Problem Statement Find all employees whose first name contains an odd number of characters.

[!tip] Concepts Tested CHAR_LENGTH(), MOD()

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, CHAR_LENGTH(first_name) AS name_len 
FROM employees 
WHERE MOD(CHAR_LENGTH(first_name), 2) = 1 
LIMIT 15;
Explanation: MOD(val, 2) = 1 checks for odd integers.

Q24. Birth Decade Distribution⌗

[!question] Problem Statement Count the total number of employees born in each decade (1950s, 1960s).

[!tip] Concepts Tested FLOOR(), YEAR(), GROUP BY

[!success]- Reveal SQL Solution
SELECT FLOOR(YEAR(birth_date) / 10) * 10 AS birth_decade, 
       COUNT(*) AS total_employees 
FROM employees 
GROUP BY birth_decade 
ORDER BY birth_decade ASC;
Explanation: Dividing by 10, applying FLOOR(), and multiplying by 10 groups years into round 10-year decade buckets.

Q25. First and Last Day of Hire Month⌗

[!question] Problem Statement For each employee, compute the first calendar day and last calendar day of the month they were hired.

[!tip] Concepts Tested DATE_FORMAT(), LAST_DAY()

[!success]- Reveal SQL Solution
SELECT emp_no, hire_date, 
       DATE_FORMAT(hire_date, '%Y-%m-01') AS first_of_month, 
       LAST_DAY(hire_date) AS last_of_month 
FROM employees 
LIMIT 15;
Explanation: DATE_FORMAT(date, '%Y-%m-01') forces the first day of the month, while LAST_DAY(date) calculates the month’s final day.

Q26. Masking Employee IDs⌗

[!question] Problem Statement Mask employee numbers showing only the last 3 digits prefixed by asterisks (e.g. ‘***123’).

[!tip] Concepts Tested RIGHT(), CONCAT()

[!success]- Reveal SQL Solution
SELECT emp_no, 
       CONCAT('***', RIGHT(emp_no, 3)) AS masked_id, 
       first_name, last_name 
FROM employees 
LIMIT 15;
Explanation: RIGHT(emp_no, 3) extracts the trailing 3 digits.

Q27. Job Title Tenure in Days⌗

[!question] Problem Statement Calculate the tenure (in days) between from_date and to_date for past title assignments.

[!tip] Concepts Tested DATEDIFF()

[!success]- Reveal SQL Solution

SELECT emp_no, title, from_date, to_date, 
       DATEDIFF(to_date, from_date) AS tenure_in_days 
FROM titles 
WHERE to_date <> '9999-01-01' 
ORDER BY tenure_in_days DESC 
LIMIT 15;

Explanation: DATEDIFF(d1, d2) computes d1 - d2 in whole calendar days.

Q28. Shared Birthday Analysis⌗

[!question] Problem Statement Find which calendar days (month and day) have the highest count of employee birthdays.

[!tip] Concepts Tested MONTH(), DAY(), GROUP BY, HAVING

[!success]- Reveal SQL Solution

SELECT MONTH(birth_date) AS b_month, 
       DAY(birth_date) AS b_day, 
       COUNT(*) AS employee_count 
FROM employees 
GROUP BY b_month, b_day 
HAVING employee_count > 1 
ORDER BY employee_count DESC 
LIMIT 10;

Explanation: Groups by month and day together, ignoring birth year.

Q29. Employees Hired on Their Birthday⌗

[!question] Problem Statement Identify employees who were hired on the exact anniversary of their birthday.

[!tip] Concepts Tested Comparing MONTH() and DAY() across two columns

[!success]- Reveal SQL Solution
SELECT emp_no, first_name, last_name, birth_date, hire_date 
FROM employees 
WHERE MONTH(birth_date) = MONTH(hire_date) 
  AND DAY(birth_date) = DAY(hire_date) 
LIMIT 15;
Explanation: Compares the calendar month and day components between birth_date and hire_date.

Q30. Sanitizing Department Names⌗

[!question] Problem Statement Replace all spaces and hyphens in department names with underscores.

[!tip] Concepts Tested Nested REPLACE() calls

[!success]- Reveal SQL Solution
SELECT dept_no, dept_name, 
       REPLACE(REPLACE(dept_name, ' ', '_'), '-', '_') AS sanitized_name 
FROM departments;
Explanation: Chains REPLACE() calls to sanitize string characters sequentially.

🟢 Part 3: Aggregations, GROUP BY & HAVING⌗

Focus: Descriptive statistics, multi-tier grouping, aggregate filters, and ROLLUP.

Q31. Gender Headcount and Percentage⌗

[!question] Problem Statement Find the total count and overall percentage of male and female employees.

[!tip] Concepts Tested COUNT(), Subquery for percentage calculation

[!success]- Reveal SQL Solution
SELECT gender, 
       COUNT(*) AS total_employees, 
       ROUND(COUNT(*) * 100.0 / (SELECT COUNT(*) FROM employees), 2) AS percentage 
FROM employees 
GROUP BY gender;
Explanation: Uses a scalar subquery (SELECT COUNT(*) FROM employees) in the expression to compute proportional percentage.

Q32. Hires Per Calendar Year⌗

[!question] Problem Statement Count the total number of employees hired each year, ordered chronologically.

[!tip] Concepts Tested YEAR(), GROUP BY, ORDER BY

[!success]- Reveal SQL Solution
SELECT YEAR(hire_date) AS hire_year, 
       COUNT(*) AS hires_count 
FROM employees 
GROUP BY hire_year 
ORDER BY hire_year ASC;
Explanation: Aggregates total hires grouped by the extracted hire_year.

Q33. Current Overall Salary Statistics⌗

[!question] Problem Statement Calculate the overall average, minimum, maximum, and total current compensation across all active employees.

[!tip] Concepts Tested AVG(), MIN(), MAX(), SUM(), ROUND()

[!success]- Reveal SQL Solution

SELECT COUNT(*) AS active_payroll_count, 
       MIN(salary) AS min_salary, 
       MAX(salary) AS max_salary, 
       ROUND(AVG(salary), 2) AS avg_salary, 
       SUM(salary) AS total_payroll 
FROM salaries 
WHERE to_date = '9999-01-01';

Explanation: Calculates foundational descriptive statistics over the active salary population.

Q34. Headcount by Active Job Title⌗

[!question] Problem Statement Find the total number of employees currently holding each job title.

[!tip] Concepts Tested GROUP BY, Active filter

[!success]- Reveal SQL Solution

SELECT title, 
       COUNT(*) AS active_holders 
FROM titles 
WHERE to_date = '9999-01-01' 
GROUP BY title 
ORDER BY active_holders DESC;

Explanation: Aggregates active employees per title.

Q35. Departments with Over 15,000 Active Employees⌗

[!question] Problem Statement Find department IDs that currently have more than 15,000 active employees assigned.

[!tip] Concepts Tested GROUP BY, HAVING

[!success]- Reveal SQL Solution

SELECT dept_no, 
       COUNT(*) AS active_headcount 
FROM dept_emp 
WHERE to_date = '9999-01-01' 
GROUP BY dept_no 
HAVING active_headcount > 15000 
ORDER BY active_headcount DESC;

Explanation: HAVING filters group-level aggregate counts after GROUP BY execution.

Q36. Average Current Salary by Department⌗

[!question] Problem Statement Calculate the average current salary for each department.

[!tip] Concepts Tested 3-table JOIN, GROUP BY, AVG()

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name, 
       ROUND(AVG(s.salary), 2) AS average_salary 
FROM departments d 
JOIN dept_emp de ON d.dept_no = de.dept_no 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
GROUP BY d.dept_no, d.dept_name 
ORDER BY average_salary DESC;

Explanation: Joins departments, dept_emp, and salaries filtering both membership and compensation to active terms.

Q37. Titles with Average Salary Above $75,000⌗

[!question] Problem Statement List all job titles whose current average salary exceeds $75,000.

[!tip] Concepts Tested JOIN, GROUP BY, HAVING

[!success]- Reveal SQL Solution

SELECT t.title, 
       ROUND(AVG(s.salary), 2) AS avg_title_salary 
FROM titles t 
JOIN salaries s ON t.emp_no = s.emp_no 
WHERE t.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
GROUP BY t.title 
HAVING avg_title_salary > 75000 
ORDER BY avg_title_salary DESC;

Explanation: Filters aggregated salary averages using HAVING avg_title_salary > 75000.

Q38. Employees with the Most Salary Raises⌗

[!question] Problem Statement Find employees who have received the highest number of historical salary adjustments.

[!tip] Concepts Tested COUNT(*) - 1, GROUP BY

[!success]- Reveal SQL Solution
SELECT emp_no, 
       COUNT(*) - 1 AS raise_count 
FROM salaries 
GROUP BY emp_no 
HAVING raise_count >= 15 
ORDER BY raise_count DESC 
LIMIT 10;
Explanation: An employee with $N$ salary records has received $N - 1$ salary increases/adjustments.

Q39. Department Salary Spread (> $50,000)⌗

[!question] Problem Statement Find departments where the spread (max minus min current salary) exceeds $50,000.

[!tip] Concepts Tested MAX() - MIN(), HAVING

[!success]- Reveal SQL Solution

SELECT d.dept_name, 
       MIN(s.salary) AS min_sal, 
       MAX(s.salary) AS max_sal, 
       (MAX(s.salary) - MIN(s.salary)) AS salary_spread 
FROM departments d 
JOIN dept_emp de ON d.dept_no = de.dept_no 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
GROUP BY d.dept_name 
HAVING salary_spread > 50000 
ORDER BY salary_spread DESC;

Explanation: Evaluates the wage disparity within each department.

Q40. High Volume Hiring Years (> 1,000 Hires)⌗

[!question] Problem Statement Identify the years in which the company hired more than 1,000 employees.

[!tip] Concepts Tested YEAR(), COUNT(), HAVING

[!success]- Reveal SQL Solution

SELECT YEAR(hire_date) AS hire_year, 
       COUNT(*) AS total_hires 
FROM employees 
GROUP BY hire_year 
HAVING total_hires > 1000 
ORDER BY total_hires DESC;

Explanation: Filters years based on aggregated recruitment volume.

Q41. Total Active Payroll by Department⌗

[!question] Problem Statement Calculate the total annualized payroll budget for each department for all active employees.

[!tip] Concepts Tested SUM(), GROUP BY

[!success]- Reveal SQL Solution

SELECT d.dept_name, 
       SUM(s.salary) AS total_payroll_expenditure 
FROM departments d 
JOIN dept_emp de ON d.dept_no = de.dept_no 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
GROUP BY d.dept_name 
ORDER BY total_payroll_expenditure DESC;

Explanation: Sums active individual salaries grouped by department name.

Q42. Employees with 3 or More Distinct Titles⌗

[!question] Problem Statement Identify all employees who have held at least 3 distinct job titles in their career.

[!tip] Concepts Tested COUNT(DISTINCT title), HAVING

[!success]- Reveal SQL Solution
SELECT emp_no, 
       COUNT(DISTINCT title) AS distinct_titles_count 
FROM titles 
GROUP BY emp_no 
HAVING distinct_titles_count >= 3 
ORDER BY distinct_titles_count DESC 
LIMIT 15;
Explanation: COUNT(DISTINCT title) prevents multiple records with the same title from inflating the count.

Q43. Average Starting Salary by Birth Decade⌗

[!question] Problem Statement Calculate the average initial starting salary for employees grouped by birth decade.

[!tip] Concepts Tested Subquery for initial salary + GROUP BY

[!success]- Reveal SQL Solution

WITH InitialSalaries AS (
    SELECT emp_no, salary AS starting_salary
    FROM salaries
    WHERE (emp_no, from_date) IN (
        SELECT emp_no, MIN(from_date)
        FROM salaries
        GROUP BY emp_no
    )
)
SELECT FLOOR(YEAR(e.birth_date) / 10) * 10 AS birth_decade,
       ROUND(AVG(i.starting_salary), 2) AS avg_initial_salary
FROM employees e
JOIN InitialSalaries i ON e.emp_no = i.emp_no
GROUP BY birth_decade
ORDER BY birth_decade ASC;

Explanation: Uses a CTE to isolate each employee’s earliest salary record, then computes decade averages.

Q44. Department with the Highest Turnover of Managers⌗

[!question] Problem Statement Find the department that has had the highest total number of managers across history.

[!tip] Concepts Tested COUNT(*), ORDER BY DESC, LIMIT 1

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name, 
       COUNT(*) AS total_managers_count 
FROM departments d 
JOIN dept_manager dm ON d.dept_no = dm.dept_no 
GROUP BY d.dept_no, d.dept_name 
ORDER BY total_managers_count DESC 
LIMIT 1;

Explanation: Counts historical manager rows per department.

Q45. Department & Gender Subtotals (WITH ROLLUP)⌗

[!question] Problem Statement Compute employee count subtotals and grand totals across department and gender using WITH ROLLUP.

[!tip] Concepts Tested GROUP BY ... WITH ROLLUP, IFNULL()

[!success]- Reveal SQL Solution

SELECT IFNULL(d.dept_name, '-- COMPANY TOTAL --') AS department, 
       IFNULL(e.gender, '-- ALL GENDERS --') AS gender, 
       COUNT(*) AS employee_count 
FROM employees e 
JOIN dept_emp de ON e.emp_no = de.emp_no 
JOIN departments d ON de.dept_no = d.dept_no 
WHERE de.to_date = '9999-01-01' 
GROUP BY d.dept_name, e.gender WITH ROLLUP;

Explanation: WITH ROLLUP adds hierarchical subtotal and grand total rows to the grouped result.

🟢 Part 4: Relational Joins Mastery⌗

Focus: Multi-table joins, self-joins, anti-joins, and relational modeling.

Q46. Current Employee Title and Salary⌗

[!question] Problem Statement List each employee’s full name, current job title, and current salary.

[!tip] Concepts Tested Multi-table INNER JOIN

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       t.title, 
       s.salary 
FROM employees e 
JOIN titles t ON e.emp_no = t.emp_no 
JOIN salaries s ON e.emp_no = s.emp_no 
WHERE t.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
LIMIT 20;

Explanation: Joins employees, titles, and salaries filtering to active records.

Q47. Employees and Their Current Department⌗

[!question] Problem Statement Retrieve each employee’s name along with the name of the department they currently work in.

[!tip] Concepts Tested 3-Table INNER JOIN

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       d.dept_name 
FROM employees e 
JOIN dept_emp de ON e.emp_no = de.emp_no 
JOIN departments d ON de.dept_no = d.dept_no 
WHERE de.to_date = '9999-01-01' 
LIMIT 20;

Explanation: Resolves the Many-to-Many bridge table dept_emp between employees and departments.

Q48. All Current Department Managers⌗

[!question] Problem Statement Display the full name, department name, and start date of all current department managers.

[!tip] Concepts Tested JOIN on dept_manager

[!success]- Reveal SQL Solution

SELECT dm.dept_no, d.dept_name, 
       CONCAT(e.first_name, ' ', e.last_name) AS manager_name, 
       dm.from_date AS managing_since 
FROM dept_manager dm 
JOIN departments d ON dm.dept_no = d.dept_no 
JOIN employees e ON dm.emp_no = e.emp_no 
WHERE dm.to_date = '9999-01-01';

Explanation: Filters dept_manager on to_date = '9999-01-01' and pulls manager demographic details.

Q49. Employees Alongside Their Current Manager⌗

[!question] Problem Statement Display each employee alongside the full name of their department’s current manager.

[!tip] Concepts Tested Self/Multi-Join involving dept_emp and dept_manager

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS employee_name, 
       d.dept_name, 
       CONCAT(m.first_name, ' ', m.last_name) AS department_manager 
FROM employees e 
JOIN dept_emp de ON e.emp_no = de.emp_no AND de.to_date = '9999-01-01' 
JOIN departments d ON de.dept_no = d.dept_no 
JOIN dept_manager dm ON de.dept_no = dm.dept_no AND dm.to_date = '9999-01-01' 
JOIN employees m ON dm.emp_no = m.emp_no 
LIMIT 20;

Explanation: Joins the employees table twice (once aliased as e for the worker, once as m for their manager).

Q50. Departments Without an Active Manager⌗

[!question] Problem Statement Verify whether any departments currently lack an active manager assignment.

[!tip] Concepts Tested LEFT JOIN ... WHERE ... IS NULL (Anti-Join)

[!success]- Reveal SQL Solution
SELECT d.dept_no, d.dept_name 
FROM departments d 
LEFT JOIN dept_manager dm 
       ON d.dept_no = dm.dept_no 
      AND dm.to_date = '9999-01-01' 
WHERE dm.emp_no IS NULL;
Explanation: An anti-join pattern that flags any department with no active matching dept_manager row.

Q51. Employees Who Changed Departments⌗

[!question] Problem Statement Find all employees who have worked in more than one department throughout their career.

[!tip] Concepts Tested GROUP BY, HAVING COUNT(dept_no) > 1

[!success]- Reveal SQL Solution

SELECT de.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       COUNT(de.dept_no) AS departments_worked 
FROM dept_emp de 
JOIN employees e ON de.emp_no = e.emp_no 
GROUP BY de.emp_no, full_name 
HAVING departments_worked > 1 
ORDER BY departments_worked DESC 
LIMIT 15;

Explanation: Groups by employee to detect career internal transfers across department boundaries.

Q52. Senior Engineers Earning Less Than Average Engineers⌗

[!question] Problem Statement Find all ‘Senior Engineers’ whose current salary is lower than the current average salary of ‘Engineers’.

[!tip] Concepts Tested Subquery + Multi-table JOIN

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       s.salary 
FROM employees e 
JOIN titles t ON e.emp_no = t.emp_no 
JOIN salaries s ON e.emp_no = s.emp_no 
WHERE t.title = 'Senior Engineer' 
  AND t.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
  AND s.salary < (
      SELECT AVG(s2.salary) 
      FROM titles t2 
      JOIN salaries s2 ON t2.emp_no = s2.emp_no 
      WHERE t2.title = 'Engineer' 
        AND t2.to_date = '9999-01-01' 
        AND s2.to_date = '9999-01-01'
  ) 
LIMIT 20;

Explanation: Compares individual compensation against a benchmark aggregate computed for a lower title tier.

Q53. Longest-Serving Manager per Department⌗

[!question] Problem Statement For each department, find the manager who held the leadership position for the longest duration.

[!tip] Concepts Tested CTE, Window Function DENSE_RANK(), DATEDIFF()

[!success]- Reveal SQL Solution

WITH ManagerTenures AS (
    SELECT dm.dept_no, dm.emp_no,
           DATEDIFF(IF(dm.to_date = '9999-01-01', CURDATE(), dm.to_date), dm.from_date) AS tenure_days,
           DENSE_RANK() OVER (
               PARTITION BY dm.dept_no 
               ORDER BY DATEDIFF(IF(dm.to_date = '9999-01-01', CURDATE(), dm.to_date), dm.from_date) DESC
           ) AS rank_order
    FROM dept_manager dm
)
SELECT d.dept_name, 
       CONCAT(e.first_name, ' ', e.last_name) AS longest_serving_manager, 
       mt.tenure_days 
FROM ManagerTenures mt 
JOIN departments d ON mt.dept_no = d.dept_no 
JOIN employees e ON mt.emp_no = e.emp_no 
WHERE mt.rank_order = 1;

Explanation: Calculates tenure in days (substituting CURDATE() for active terms) and ranks tenures per department.

Q54. Employees Earning More Than Their Current Manager⌗

[!question] Problem Statement Find all active employees who earn a higher current salary than their department’s current manager.

[!tip] Concepts Tested Double Join on dept_manager & salaries

[!success]- Reveal SQL Solution

SELECT e.emp_no AS employee_id, 
       CONCAT(e.first_name, ' ', e.last_name) AS employee_name, 
       s_emp.salary AS employee_salary, 
       CONCAT(m.first_name, ' ', m.last_name) AS manager_name, 
       s_mgr.salary AS manager_salary, 
       d.dept_name 
FROM dept_emp de 
JOIN salaries s_emp ON de.emp_no = s_emp.emp_no AND s_emp.to_date = '9999-01-01' 
JOIN employees e ON de.emp_no = e.emp_no 
JOIN departments d ON de.dept_no = d.dept_no 
JOIN dept_manager dm ON de.dept_no = dm.dept_no AND dm.to_date = '9999-01-01' 
JOIN salaries s_mgr ON dm.emp_no = s_mgr.emp_no AND s_mgr.to_date = '9999-01-01' 
JOIN employees m ON dm.emp_no = m.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s_emp.salary > s_mgr.salary 
LIMIT 20;

Explanation: Classic organizational interview question comparing staff compensation against management compensation.

Q55. Empty Departments Check⌗

[!question] Problem Statement Check if any department currently has zero active employee assignments.

[!tip] Concepts Tested LEFT JOIN ... IS NULL

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name 
FROM departments d 
LEFT JOIN dept_emp de 
       ON d.dept_no = de.dept_no 
      AND de.to_date = '9999-01-01' 
WHERE de.emp_no IS NULL;

Explanation: Detects dormant or empty departments.

Q56. Staff with Same Title as Their Department Manager⌗

[!question] Problem Statement Find active employees who hold the exact same job title as their current department manager.

[!tip] Concepts Tested JOIN comparing two title rows

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS employee, 
       t_emp.title, 
       d.dept_name 
FROM dept_emp de 
JOIN dept_manager dm ON de.dept_no = dm.dept_no 
JOIN titles t_emp ON de.emp_no = t_emp.emp_no 
JOIN titles t_mgr ON dm.emp_no = t_mgr.emp_no 
JOIN employees e ON de.emp_no = e.emp_no 
JOIN departments d ON de.dept_no = d.dept_no 
WHERE de.to_date = '9999-01-01' 
  AND dm.to_date = '9999-01-01' 
  AND t_emp.to_date = '9999-01-01' 
  AND t_mgr.to_date = '9999-01-01' 
  AND de.emp_no <> dm.emp_no 
  AND t_emp.title = t_mgr.title 
LIMIT 20;

Explanation: Excludes the manager themselves (de.emp_no <> dm.emp_no) and checks for title identity.

Q57. Department Peers Hired on the Same Day⌗

[!question] Problem Statement Find pairs of employees who work in the same active department and were hired on the exact same date.

[!tip] Concepts Tested Self-Join with Inequality (e1.emp_no < e2.emp_no)

[!success]- Reveal SQL Solution

SELECT e1.emp_no AS employee_1, 
       e2.emp_no AS employee_2, 
       de1.dept_no, 
       e1.hire_date 
FROM employees e1 
JOIN employees e2 
  ON e1.hire_date = e2.hire_date 
 AND e1.emp_no < e2.emp_no 
JOIN dept_emp de1 
  ON e1.emp_no = de1.emp_no AND de1.to_date = '9999-01-01' 
JOIN dept_emp de2 
  ON e2.emp_no = de2.emp_no AND de2.to_date = '9999-01-01' 
 AND de1.dept_no = de2.dept_no 
LIMIT 20;

Explanation: e1.emp_no < e2.emp_no prevents pairing an employee with themselves and removes reverse duplicate pairs (A, B) vs (B, A).

Q58. Transfers from Marketing (d001) to Finance (d002)⌗

[!question] Problem Statement Find all employees who transferred directly from Marketing (‘d001’) to Finance (‘d002’).

[!tip] Concepts Tested Self-join on dept_emp with chronological order

[!success]- Reveal SQL Solution

SELECT de1.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       de1.from_date AS mkt_start, 
       de2.from_date AS fin_start 
FROM dept_emp de1 
JOIN dept_emp de2 ON de1.emp_no = de2.emp_no 
JOIN employees e ON de1.emp_no = e.emp_no 
WHERE de1.dept_no = 'd001' 
  AND de2.dept_no = 'd002' 
  AND de1.from_date < de2.from_date;

Explanation: Matches two department records for the same employee where the marketing tenure preceded the finance tenure.

Q59. Starting Salary for Each Title Held⌗

[!question] Problem Statement Retrieve each employee’s salary when they first obtained each of their historical titles.

[!tip] Concepts Tested JOIN on emp_no and matching from_date

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       t.title, 
       s.salary AS initial_title_salary, 
       t.from_date 
FROM employees e 
JOIN titles t ON e.emp_no = t.emp_no 
JOIN salaries s ON e.emp_no = s.emp_no 
               AND t.from_date = s.from_date 
LIMIT 20;

Explanation: Synchronizes promotion dates with simultaneous compensation updates.

Q60. Unrepresented Title and Department Combinations⌗

[!question] Problem Statement Generate a Cartesian product of all departments and distinct titles to find combinations with zero active employees.

[!tip] Concepts Tested CROSS JOIN, Anti-Join

[!success]- Reveal SQL Solution

SELECT d.dept_name, t.title 
FROM departments d 
CROSS JOIN (SELECT DISTINCT title FROM titles) t 
LEFT JOIN (
    SELECT DISTINCT de.dept_no, ti.title 
    FROM dept_emp de 
    JOIN titles ti ON de.emp_no = ti.emp_no 
    WHERE de.to_date = '9999-01-01' 
      AND ti.to_date = '9999-01-01'
) active_combos 
  ON d.dept_no = active_combos.dept_no 
 AND t.title = active_combos.title 
WHERE active_combos.dept_no IS NULL;

Explanation: Generates all theoretical combinations with CROSS JOIN and subtracts active reality using an anti-join.

🟢 Part 5: Subqueries, CTEs & Set Operations⌗

Focus: Scalar, correlated subqueries, EXISTS, UNION, and common table expressions.

Q61. Single Highest Current Earner (No LIMIT)⌗

[!question] Problem Statement Find the employee who earns the single highest current salary without using ORDER BY ... LIMIT.

[!tip] Concepts Tested Subquery with MAX()

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       s.salary 
FROM employees e 
JOIN salaries s ON e.emp_no = s.emp_no 
WHERE s.to_date = '9999-01-01' 
  AND s.salary = (
      SELECT MAX(salary) 
      FROM salaries 
      WHERE to_date = '9999-01-01'
  );

Explanation: Standard interview test forbidding LIMIT and testing subquery knowledge with MAX().

Q62. Employees Earning Above Company Average⌗

[!question] Problem Statement Find all active employees whose current salary is strictly above the overall company average.

[!tip] Concepts Tested Scalar Subquery in WHERE

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       s.salary 
FROM employees e 
JOIN salaries s ON e.emp_no = s.emp_no 
WHERE s.to_date = '9999-01-01' 
  AND s.salary > (
      SELECT AVG(salary) 
      FROM salaries 
      WHERE to_date = '9999-01-01'
  ) 
LIMIT 20;

Explanation: Compares individual compensation against a scalar aggregate subquery.

Q63. Departments with High Earners (> $100k) Using EXISTS⌗

[!question] Problem Statement Using EXISTS, list all departments that have at least one active employee earning over $100,000.

[!tip] Concepts Tested Correlated Subquery with EXISTS

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name 
FROM departments d 
WHERE EXISTS (
    SELECT 1 
    FROM dept_emp de 
    JOIN salaries s ON de.emp_no = s.emp_no 
    WHERE de.dept_no = d.dept_no 
      AND de.to_date = '9999-01-01' 
      AND s.to_date = '9999-01-01' 
      AND s.salary > 100000
);

Explanation: EXISTS terminates as soon as a single matching record is discovered, making it highly efficient.

Q64. Employees Without a Pay Raise (NOT EXISTS)⌗

[!question] Problem Statement Using NOT EXISTS, find all employees who have only ever had exactly 1 salary record (zero raises).

[!tip] Concepts Tested NOT EXISTS with Correlation

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       e.hire_date 
FROM employees e 
WHERE NOT EXISTS (
    SELECT 1 
    FROM salaries s 
    WHERE s.emp_no = e.emp_no 
      AND s.from_date > e.hire_date
) 
LIMIT 20;

Explanation: Finds workers who have never had a subsequent salary update record after their initial hire date.

Q65. Employees Earning Above Their Department Average⌗

[!question] Problem Statement Find all employees who earn more than the average salary of their own respective department.

[!tip] Concepts Tested Correlated Subquery in WHERE

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       de.dept_no, 
       s.salary 
FROM employees e 
JOIN dept_emp de ON e.emp_no = de.emp_no AND de.to_date = '9999-01-01' 
JOIN salaries s ON e.emp_no = s.emp_no AND s.to_date = '9999-01-01' 
WHERE s.salary > (
    SELECT AVG(s2.salary) 
    FROM dept_emp de2 
    JOIN salaries s2 ON de2.emp_no = s2.emp_no 
    WHERE de2.dept_no = de.dept_no 
      AND de2.to_date = '9999-01-01' 
      AND s2.to_date = '9999-01-01'
) 
LIMIT 20;

Explanation: Correlates the subquery on de2.dept_no = de.dept_no to evaluate averages for each row’s department.

Q66. Second Highest Current Salary⌗

[!question] Problem Statement Find the 2nd highest distinct current salary in the company using a subquery.

[!tip] Concepts Tested Subquery with MAX()

[!success]- Reveal SQL Solution
SELECT MAX(salary) AS second_highest_salary 
FROM salaries 
WHERE to_date = '9999-01-01' 
  AND salary < (
      SELECT MAX(salary) 
      FROM salaries 
      WHERE to_date = '9999-01-01'
  );
Explanation: The highest value that is strictly less than the overall maximum is the second highest value.

Q67. The Nth Highest Salary (Generic Pattern)⌗

[!question] Problem Statement Write a generic correlated subquery to find the Nth highest current salary (e.g. N = 3).

[!tip] Concepts Tested Correlated Subquery counting higher distinct values

[!success]- Reveal SQL Solution

SELECT DISTINCT s1.salary AS third_highest_salary 
FROM salaries s1 
WHERE to_date = '9999-01-01' 
  AND 2 = (
      SELECT COUNT(DISTINCT s2.salary) 
      FROM salaries s2 
      WHERE s2.to_date = '9999-01-01' 
        AND s2.salary > s1.salary
  );

Explanation: The $N$-th highest value has exactly $N-1$ distinct values greater than itself.

Q68. Employees Holding Both Senior Engineer and Technique Leader⌗

[!question] Problem Statement Find employees who have held both ‘Senior Engineer’ and ‘Technique Leader’ titles.

[!tip] Concepts Tested INTERSECT or IN Subquery

[!success]- Reveal SQL Solution

SELECT emp_no 
FROM titles 
WHERE title = 'Senior Engineer' 
  AND emp_no IN (
      SELECT emp_no 
      FROM titles 
      WHERE title = 'Technique Leader'
  ) 
LIMIT 20;

Explanation: Simulates a set intersection identifying individuals appearing in both title sets.

Q69. Initial Salary Higher Than Department Average Initial Salary⌗

[!question] Problem Statement Find employees whose starting salary was strictly greater than their department’s average starting salary.

[!tip] Concepts Tested CTEs with modular aggregation

[!success]- Reveal SQL Solution

WITH FirstSalaries AS (
    SELECT emp_no, salary AS start_salary, from_date
    FROM salaries
    WHERE (emp_no, from_date) IN (
        SELECT emp_no, MIN(from_date) FROM salaries GROUP BY emp_no
    )
),
DeptStartAverages AS (
    SELECT de.dept_no, AVG(fs.start_salary) AS dept_avg_start
    FROM dept_emp de
    JOIN FirstSalaries fs ON de.emp_no = fs.emp_no
    GROUP BY de.dept_no
)
SELECT e.emp_no, de.dept_no, fs.start_salary, ROUND(dsa.dept_avg_start, 2) AS dept_avg
FROM employees e
JOIN dept_emp de ON e.emp_no = de.emp_no AND de.to_date = '9999-01-01'
JOIN FirstSalaries fs ON e.emp_no = fs.emp_no
JOIN DeptStartAverages dsa ON de.dept_no = dsa.dept_no
WHERE fs.start_salary > dsa.dept_avg_start
LIMIT 20;

Explanation: Modularizes starting salary discovery and department benchmark calculations using CTEs.

Q70. Managers and $120k+ Earners (UNION)⌗

[!question] Problem Statement Combine the distinct list of all current managers and all employees earning over $120,000.

[!tip] Concepts Tested UNION (Set Union)

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       'Department Manager' AS qualifying_reason 
FROM dept_manager dm 
JOIN employees e ON dm.emp_no = e.emp_no 
WHERE dm.to_date = '9999-01-01' 

UNION 

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       'High Earner (>120k)' AS qualifying_reason 
FROM salaries s 
JOIN employees e ON s.emp_no = e.emp_no 
WHERE s.to_date = '9999-01-01' 
  AND s.salary > 120000 
ORDER BY emp_no ASC 
LIMIT 25;

Explanation: UNION combines two distinct query sets and eliminates duplicate records.

Q71. Employees in the Highest-Paying Department⌗

[!question] Problem Statement Find all active employees who work in the department that has the highest average salary.

[!tip] Concepts Tested Subquery with ORDER BY ... LIMIT 1

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
       de.dept_no 
FROM employees e 
JOIN dept_emp de ON e.emp_no = de.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND de.dept_no = (
      SELECT de2.dept_no 
      FROM dept_emp de2 
      JOIN salaries s2 ON de2.emp_no = s2.emp_no 
      WHERE de2.to_date = '9999-01-01' 
        AND s2.to_date = '9999-01-01' 
      GROUP BY de2.dept_no 
      ORDER BY AVG(s2.salary) DESC 
      LIMIT 1
  ) 
LIMIT 20;

Explanation: Finds the top department ID via a subquery, then filters active employees by that ID.

Q72. Salary Delta from Company Average⌗

[!question] Problem Statement Display each active employee’s salary alongside the exact numerical difference from the overall company average.

[!tip] Concepts Tested Scalar Subquery in SELECT

[!success]- Reveal SQL Solution

SELECT emp_no, 
       salary, 
       ROUND((SELECT AVG(salary) FROM salaries WHERE to_date = '9999-01-01'), 2) AS company_avg, 
       ROUND(salary - (SELECT AVG(salary) FROM salaries WHERE to_date = '9999-01-01'), 2) AS variance_from_avg 
FROM salaries 
WHERE to_date = '9999-01-01' 
LIMIT 20;

Explanation: Evaluates a scalar aggregate subquery directly within the SELECT column list.

Q73. Employees Who Experienced a Salary Reduction⌗

[!question] Problem Statement Identify employees who had a salary decrease from one pay record to the next.

[!tip] Concepts Tested Correlated Subquery or Self-Join

[!success]- Reveal SQL Solution
SELECT DISTINCT s1.emp_no 
FROM salaries s1 
JOIN salaries s2 
  ON s1.emp_no = s2.emp_no 
 AND s1.to_date = s2.from_date 
WHERE s2.salary < s1.salary 
LIMIT 20;
Explanation: Matches consecutive salary intervals (s1.to_date = s2.from_date) and checks if the new salary s2 is strictly lower than s1.

Q74. Employees Hired After Their Current Manager⌗

[!question] Problem Statement Find active employees who were hired on a date after their department’s current manager was hired.

[!tip] Concepts Tested Multi-table comparison

[!success]- Reveal SQL Solution

SELECT e.emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS employee, 
       e.hire_date AS employee_hired, 
       CONCAT(m.first_name, ' ', m.last_name) AS manager, 
       m.hire_date AS manager_hired 
FROM dept_emp de 
JOIN employees e ON de.emp_no = e.emp_no 
JOIN dept_manager dm ON de.dept_no = dm.dept_no AND dm.to_date = '9999-01-01' 
JOIN employees m ON dm.emp_no = m.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND e.hire_date > m.hire_date 
LIMIT 20;

Explanation: Compares the hire_date attributes between employee and department manager.

Q75. Departments Where Every Employee Earns Over $40k⌗

[!question] Problem Statement Find all department IDs where every single active employee earns over $40,000 (no one earns <= $40,000).

[!tip] Concepts Tested NOT EXISTS Double Negative

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name 
FROM departments d 
WHERE NOT EXISTS (
    SELECT 1 
    FROM dept_emp de 
    JOIN salaries s ON de.emp_no = s.emp_no 
    WHERE de.dept_no = d.dept_no 
      AND de.to_date = '9999-01-01' 
      AND s.to_date = '9999-01-01' 
      AND s.salary <= 40000
);

Explanation: Universal quantification in SQL (‘all X satisfy Y’) is expressed via universal negative (’there exists no X that violates Y’).

🟢 Part 6: Window Functions & Analytical SQL⌗

Focus: ROW_NUMBER, DENSE_RANK, LAG, LEAD, NTILE, and running aggregates.

Q76. Department Salary Ranking (DENSE_RANK)⌗

[!question] Problem Statement Rank active employees by salary within their respective department without gaps for ties.

[!tip] Concepts Tested DENSE_RANK() OVER (PARTITION BY ... ORDER BY ...)

[!success]- Reveal SQL Solution

SELECT de.dept_no, 
       s.emp_no, 
       s.salary, 
       DENSE_RANK() OVER (
           PARTITION BY de.dept_no 
           ORDER BY s.salary DESC
       ) AS department_salary_rank 
FROM dept_emp de 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
LIMIT 25;

Explanation: DENSE_RANK() assigns ranks 1, 2, 2, 3 without skipping rank numbers when ties occur.

Q77. Top 3 Highest-Paid Employees per Department⌗

[!question] Problem Statement Retrieve the top 3 highest-earning employees in every department.

[!tip] Concepts Tested CTE + ROW_NUMBER() Window Function

[!success]- Reveal SQL Solution

WITH DepartmentRankings AS (
    SELECT de.dept_no, 
           d.dept_name, 
           e.emp_no, 
           CONCAT(e.first_name, ' ', e.last_name) AS full_name, 
           s.salary, 
           ROW_NUMBER() OVER (
               PARTITION BY de.dept_no 
               ORDER BY s.salary DESC
           ) AS rank_in_dept 
    FROM dept_emp de 
    JOIN departments d ON de.dept_no = d.dept_no 
    JOIN employees e ON de.emp_no = e.emp_no 
    JOIN salaries s ON de.emp_no = s.emp_no 
    WHERE de.to_date = '9999-01-01' 
      AND s.to_date = '9999-01-01'
)
SELECT dept_no, dept_name, full_name, salary, rank_in_dept 
FROM DepartmentRankings 
WHERE rank_in_dept <= 3 
ORDER BY dept_no, rank_in_dept;

Explanation: Classic SQL interview archetype: Partition by group, order by metric, filter rank <= N via CTE.

Q78. Running Total (Cumulative Sum) of Payroll⌗

[!question] Problem Statement Calculate the running cumulative sum of active payroll ordered by employee ID.

[!tip] Concepts Tested SUM() OVER (ORDER BY ...)

[!success]- Reveal SQL Solution

SELECT emp_no, 
       salary, 
       SUM(salary) OVER (
           ORDER BY emp_no ASC 
           ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
       ) AS cumulative_payroll 
FROM salaries 
WHERE to_date = '9999-01-01' 
LIMIT 20;

Explanation: Computes cumulative rolling sum across rows from the start of the partition through the current row.

Q79. Salary Raise Percentage per Employee (LAG)⌗

[!question] Problem Statement For each employee, display their current salary, previous salary, and the percentage raise received.

[!tip] Concepts Tested LAG() OVER (PARTITION BY ... ORDER BY ...)

[!success]- Reveal SQL Solution

WITH SalaryHistory AS (
    SELECT emp_no, 
           salary, 
           from_date, 
           LAG(salary, 1) OVER (
               PARTITION BY emp_no 
               ORDER BY from_date ASC
           ) AS prev_salary 
    FROM salaries
)
SELECT emp_no, 
       from_date, 
       prev_salary, 
       salary AS current_term_salary, 
       ROUND(((salary - prev_salary) / prev_salary) * 100, 2) AS percentage_increase 
FROM SalaryHistory 
WHERE prev_salary IS NOT NULL 
LIMIT 20;

Explanation: LAG(col, 1) accesses data from the previous row within the partition.

Q80. Look Ahead to Next Salary (LEAD)⌗

[!question] Problem Statement Using LEAD(), compare each historical salary with the next one to flag any pay reductions.

[!tip] Concepts Tested LEAD() OVER (...)

[!success]- Reveal SQL Solution

WITH NextSalaryCTE AS (
    SELECT emp_no, 
           salary, 
           from_date, 
           LEAD(salary, 1) OVER (
               PARTITION BY emp_no 
               ORDER BY from_date ASC
           ) AS next_salary 
    FROM salaries
)
SELECT emp_no, from_date, salary, next_salary, 
       (next_salary - salary) AS change_amount 
FROM NextSalaryCTE 
WHERE next_salary IS NOT NULL 
  AND next_salary < salary 
LIMIT 20;

Explanation: LEAD(col, 1) inspects the next sequential row in the window.

Q81. Salary Quartiles per Department (NTILE)⌗

[!question] Problem Statement Divide employees into 4 compensation quartiles (1=Lowest, 4=Highest) within their respective department.

[!tip] Concepts Tested NTILE(4) OVER (PARTITION BY ... ORDER BY ...)

[!success]- Reveal SQL Solution

SELECT de.dept_no, 
       s.emp_no, 
       s.salary, 
       NTILE(4) OVER (
           PARTITION BY de.dept_no 
           ORDER BY s.salary ASC
       ) AS salary_quartile 
FROM dept_emp de 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
LIMIT 25;

Explanation: NTILE(N) divides rows in each department partition into $N$ equal-sized buckets.

Q82. Max Department Salary on Every Employee Row⌗

[!question] Problem Statement Display each active employee’s salary alongside their department’s maximum salary without using GROUP BY.

[!tip] Concepts Tested MAX() OVER (PARTITION BY ...)

[!success]- Reveal SQL Solution

SELECT de.dept_no, 
       s.emp_no, 
       s.salary, 
       MAX(s.salary) OVER (PARTITION BY de.dept_no) AS dept_max_salary, 
       (MAX(s.salary) OVER (PARTITION BY de.dept_no) - s.salary) AS gap_to_top 
FROM dept_emp de 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
LIMIT 25;

Explanation: Window aggregates compute summary metrics without collapsing individual rows.

Q83. 3-Term Moving Average Salary⌗

[!question] Problem Statement Calculate the 3-term moving average salary for each employee across their compensation history.

[!tip] Concepts Tested AVG() OVER (PARTITION BY ... ROWS BETWEEN ...)

[!success]- Reveal SQL Solution

SELECT emp_no, 
       from_date, 
       salary, 
       ROUND(AVG(salary) OVER (
           PARTITION BY emp_no 
           ORDER BY from_date ASC 
           ROWS BETWEEN 2 PRECEDING AND CURRENT ROW
       ), 2) AS moving_avg_3_terms 
FROM salaries 
LIMIT 25;

Explanation: ROWS BETWEEN 2 PRECEDING AND CURRENT ROW specifies an exact 3-row moving window frame.

Q84. Gap to Top Earner (FIRST_VALUE)⌗

[!question] Problem Statement Compute the dollar gap between each employee’s salary and the highest earner in their department using FIRST_VALUE().

[!tip] Concepts Tested FIRST_VALUE() OVER (PARTITION BY ... ORDER BY ...)

[!success]- Reveal SQL Solution

SELECT de.dept_no, 
       s.emp_no, 
       s.salary, 
       FIRST_VALUE(s.salary) OVER (
           PARTITION BY de.dept_no 
           ORDER BY s.salary DESC
       ) AS top_dept_salary, 
       (FIRST_VALUE(s.salary) OVER (
           PARTITION BY de.dept_no 
           ORDER BY s.salary DESC
       ) - s.salary) AS deficit_to_top 
FROM dept_emp de 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
LIMIT 25;

Explanation: FIRST_VALUE() grabs the leading value in the sorted window partition.

Q85. Top 5th Percentile Earners (PERCENT_RANK)⌗

[!question] Problem Statement Identify employees who rank in the top 5% of earners in their department (PERCENT_RANK >= 0.95).

[!tip] Concepts Tested PERCENT_RANK() OVER (...)

[!success]- Reveal SQL Solution

WITH DepartmentPercentiles AS (
    SELECT de.dept_no, 
           s.emp_no, 
           s.salary, 
           PERCENT_RANK() OVER (
               PARTITION BY de.dept_no 
               ORDER BY s.salary ASC
           ) AS pct_rank 
    FROM dept_emp de 
    JOIN salaries s ON de.emp_no = s.emp_no 
    WHERE de.to_date = '9999-01-01' 
      AND s.to_date = '9999-01-01'
)
SELECT dept_no, emp_no, salary, ROUND(pct_rank, 4) AS percentile 
FROM DepartmentPercentiles 
WHERE pct_rank >= 0.95 
ORDER BY dept_no, salary DESC 
LIMIT 20;

Explanation: PERCENT_RANK() calculates the relative rank between 0.0 and 1.0.

Q86. Date When Peak Historical Salary Was Achieved⌗

[!question] Problem Statement For each employee, identify the exact date when they achieved their highest career salary.

[!tip] Concepts Tested Window Ranking ROW_NUMBER() over partition

[!success]- Reveal SQL Solution

WITH RankedSalaries AS (
    SELECT emp_no, salary, from_date, 
           ROW_NUMBER() OVER (
               PARTITION BY emp_no 
               ORDER BY salary DESC, from_date ASC
           ) AS rn 
    FROM salaries
)
SELECT emp_no, salary AS peak_salary, from_date AS peak_date 
FROM RankedSalaries 
WHERE rn = 1 
LIMIT 20;

Explanation: Orders compensation in descending order per employee to isolate their personal career peak.

Q87. Median Salary Earner per Department⌗

[!question] Problem Statement Find the employee whose salary represents the exact median within their department.

[!tip] Concepts Tested Window ranking comparison against COUNT(*) / 2

[!success]- Reveal SQL Solution

WITH DeptCounts AS (
    SELECT de.dept_no, s.emp_no, s.salary, 
           ROW_NUMBER() OVER (PARTITION BY de.dept_no ORDER BY s.salary ASC) AS rnk, 
           COUNT(*) OVER (PARTITION BY de.dept_no) AS total_count 
    FROM dept_emp de 
    JOIN salaries s ON de.emp_no = s.emp_no 
    WHERE de.to_date = '9999-01-01' 
      AND s.to_date = '9999-01-01'
)
SELECT dept_no, emp_no, salary AS median_salary 
FROM DeptCounts 
WHERE rnk = FLOOR((total_count + 1) / 2);

Explanation: Finds the middle row index FLOOR((total_count + 1) / 2) in the sorted salary partition.

Q88. Sequential Career Promotion Numbering⌗

[!question] Problem Statement Assign a sequential career promotion number (1, 2, 3…) to every title change an employee has experienced.

[!tip] Concepts Tested ROW_NUMBER() OVER (PARTITION BY ... ORDER BY ...)

[!success]- Reveal SQL Solution

SELECT emp_no, 
       title, 
       from_date, 
       to_date, 
       ROW_NUMBER() OVER (
           PARTITION BY emp_no 
           ORDER BY from_date ASC
       ) AS career_stage_number 
FROM titles 
LIMIT 25;

Explanation: Indexes consecutive career progression stages chronologically.

Q89. Detect Overlapping Date Ranges in Salaries⌗

[!question] Problem Statement Identify any data anomalies where an employee’s salary from_date occurs before the previous record’s to_date.

[!tip] Concepts Tested LAG() on date columns

[!success]- Reveal SQL Solution

WITH ConsecutiveSalaries AS (
    SELECT emp_no, from_date, to_date, 
           LAG(to_date, 1) OVER (
               PARTITION BY emp_no 
               ORDER BY from_date ASC
           ) AS prev_to_date 
    FROM salaries
)
SELECT emp_no, from_date, to_date, prev_to_date 
FROM ConsecutiveSalaries 
WHERE prev_to_date IS NOT NULL 
  AND from_date < prev_to_date;

Explanation: Detects invalid overlapping temporal intervals.

Q90. Top 75th Percentile Earners Across the Whole Company⌗

[!question] Problem Statement Identify active employees earning strictly in the top 25% of the overall company (CUME_DIST > 0.75).

[!tip] Concepts Tested CUME_DIST() OVER (...)

[!success]- Reveal SQL Solution

WITH OverallPercentiles AS (
    SELECT emp_no, salary, 
           CUME_DIST() OVER (ORDER BY salary ASC) AS cumulative_dist 
    FROM salaries 
    WHERE to_date = '9999-01-01'
)
SELECT emp_no, salary, ROUND(cumulative_dist, 4) AS dist 
FROM OverallPercentiles 
WHERE cumulative_dist > 0.75 
ORDER BY salary ASC 
LIMIT 20;

Explanation: CUME_DIST() evaluates cumulative distribution across the entire enterprise.

🟢 Part 7: Advanced Business Logic & Edge Cases⌗

Focus: Turnover metrics, recursive series, Gaps & Islands, pay inversion, and audit queries.

Q91. Department Turnover Rate⌗

[!question] Problem Statement Calculate the employee turnover percentage per department (departed employees vs total ever assigned).

[!tip] Concepts Tested Conditional Aggregation with CTE

[!success]- Reveal SQL Solution

WITH DepartmentTurnover AS (
    SELECT dept_no, 
           COUNT(*) AS total_ever_assigned, 
           SUM(CASE WHEN to_date <> '9999-01-01' THEN 1 ELSE 0 END) AS departed_count 
    FROM dept_emp 
    GROUP BY dept_no
)
SELECT d.dept_no, d.dept_name, 
       dt.total_ever_assigned, 
       dt.departed_count, 
       ROUND((dt.departed_count * 100.0 / dt.total_ever_assigned), 2) AS turnover_percentage 
FROM DepartmentTurnover dt 
JOIN departments d ON dt.dept_no = d.dept_no 
ORDER BY turnover_percentage DESC;

Explanation: Flags departed personnel (to_date <> '9999-01-01') and computes attrition percentage.

Q92. Continuous Calendar Series of Hires (Recursive CTE)⌗

[!question] Problem Statement Generate a complete calendar series of years from 1985 to 2000 and count employee hires, showing 0 for empty years.

[!tip] Concepts Tested WITH RECURSIVE, LEFT JOIN, COALESCE()

[!success]- Reveal SQL Solution

WITH RECURSIVE YearRange AS (
    SELECT 1985 AS cal_year
    UNION ALL
    SELECT cal_year + 1 
    FROM YearRange 
    WHERE cal_year < 2000
)
SELECT yr.cal_year, 
       COUNT(e.emp_no) AS total_hires 
FROM YearRange yr 
LEFT JOIN employees e ON yr.cal_year = YEAR(e.hire_date) 
GROUP BY yr.cal_year 
ORDER BY yr.cal_year ASC;

Explanation: Generates a synthetic timeline table on the fly to avoid omitting missing zero-count periods.

Q93. Average Promotion Duration: Engineer to Senior Engineer⌗

[!question] Problem Statement Calculate the average duration (in years) it takes for an employee to get promoted from ‘Engineer’ to ‘Senior Engineer’.

[!tip] Concepts Tested Self-join on titles with TIMESTAMPDIFF()

[!success]- Reveal SQL Solution

WITH Promotions AS (
    SELECT t1.emp_no, 
           t1.from_date AS engineer_start, 
           t2.from_date AS senior_start, 
           TIMESTAMPDIFF(MONTH, t1.from_date, t2.from_date) / 12.0 AS years_to_promo 
    FROM titles t1 
    JOIN titles t2 
      ON t1.emp_no = t2.emp_no 
     AND t1.title = 'Engineer' 
     AND t2.title = 'Senior Engineer' 
     AND t1.from_date < t2.from_date
)
SELECT ROUND(AVG(years_to_promo), 2) AS avg_years_to_promotion, 
       MIN(years_to_promo) AS fastest_promotion_years, 
       MAX(years_to_promo) AS slowest_promotion_years 
FROM Promotions;

Explanation: Calculates the promotion timeline between two specific sequential job milestones.

Q94. Manager Presiding Over the Largest Department Payroll⌗

[!question] Problem Statement Identify which manager managed the largest total current department payroll.

[!tip] Concepts Tested Multi-table JOIN, SUM(), ORDER BY DESC LIMIT 1

[!success]- Reveal SQL Solution

SELECT dm.dept_no, 
       d.dept_name, 
       dm.emp_no AS manager_emp_no, 
       CONCAT(e.first_name, ' ', e.last_name) AS manager_name, 
       SUM(s.salary) AS managed_active_payroll 
FROM dept_manager dm 
JOIN departments d ON dm.dept_no = d.dept_no 
JOIN employees e ON dm.emp_no = e.emp_no 
JOIN dept_emp de ON dm.dept_no = de.dept_no 
JOIN salaries s ON de.emp_no = s.emp_no 
WHERE dm.to_date = '9999-01-01' 
  AND de.to_date = '9999-01-01' 
  AND s.to_date = '9999-01-01' 
GROUP BY dm.dept_no, d.dept_name, dm.emp_no, manager_name 
ORDER BY managed_active_payroll DESC 
LIMIT 1;

Explanation: Aggregates total staff salaries subordinate to each active department manager.

Q95. Gaps and Islands: Continuous Title Tenures⌗

[!question] Problem Statement Detect distinct continuous islands of consecutive title periods for employees.

[!tip] Concepts Tested LAG(), SUM() OVER () Gaps and Islands pattern

[!success]- Reveal SQL Solution

WITH Step1 AS (
    SELECT emp_no, title, from_date, to_date,
           CASE 
               WHEN LAG(to_date) OVER (PARTITION BY emp_no, title ORDER BY from_date) = from_date THEN 0 
               ELSE 1 
           END AS is_new_island
    FROM titles
),
Step2 AS (
    SELECT emp_no, title, from_date, to_date,
           SUM(is_new_island) OVER (PARTITION BY emp_no, title ORDER BY from_date) AS island_id
    FROM Step1
)
SELECT emp_no, title, 
       MIN(from_date) AS consolidated_start, 
       MAX(to_date) AS consolidated_end 
FROM Step2 
GROUP BY emp_no, title, island_id 
LIMIT 20;

Explanation: Standard Gaps & Islands solution: identifies continuity breaks with LAG and groups contiguous date blocks with a cumulative sum.

Q96. Salary Inversion Detection⌗

[!question] Problem Statement Find active employees who earn less than a peer in the same department who was hired after them.

[!tip] Concepts Tested Self-Join comparing tenure vs salary

[!success]- Reveal SQL Solution

SELECT e1.emp_no AS senior_employee, 
       e1.hire_date AS senior_hire_date, 
       s1.salary AS senior_salary, 
       e2.emp_no AS junior_employee, 
       e2.hire_date AS junior_hire_date, 
       s2.salary AS junior_salary, 
       de1.dept_no 
FROM dept_emp de1 
JOIN dept_emp de2 ON de1.dept_no = de2.dept_no 
JOIN employees e1 ON de1.emp_no = e1.emp_no 
JOIN employees e2 ON de2.emp_no = e2.emp_no 
JOIN salaries s1 ON e1.emp_no = s1.emp_no 
JOIN salaries s2 ON e2.emp_no = s2.emp_no 
WHERE de1.to_date = '9999-01-01' 
  AND de2.to_date = '9999-01-01' 
  AND s1.to_date = '9999-01-01' 
  AND s2.to_date = '9999-01-01' 
  AND e1.hire_date < e2.hire_date 
  AND s1.salary < s2.salary 
LIMIT 20;

Explanation: Identifies organizational pay inversion where longer-tenured employees earn less than newer hires in the same department.

Q97. Department-by-Title Pivot Matrix⌗

[!question] Problem Statement Pivot active employee counts by title across major departments into a cross-tabulated matrix.

[!tip] Concepts Tested Conditional Aggregation (CASE WHEN ...)

[!success]- Reveal SQL Solution

SELECT d.dept_name, 
       SUM(CASE WHEN t.title = 'Senior Engineer' THEN 1 ELSE 0 END) AS sr_engineers, 
       SUM(CASE WHEN t.title = 'Engineer' THEN 1 ELSE 0 END) AS engineers, 
       SUM(CASE WHEN t.title = 'Senior Staff' THEN 1 ELSE 0 END) AS sr_staff, 
       SUM(CASE WHEN t.title = 'Staff' THEN 1 ELSE 0 END) AS staff, 
       SUM(CASE WHEN t.title = 'Technique Leader' THEN 1 ELSE 0 END) AS tech_leads 
FROM departments d 
JOIN dept_emp de ON d.dept_no = de.dept_no 
JOIN titles t ON de.emp_no = t.emp_no 
WHERE de.to_date = '9999-01-01' 
  AND t.to_date = '9999-01-01' 
GROUP BY d.dept_name 
ORDER BY d.dept_name ASC;

Explanation: Standard SQL pivot technique converting row values into dedicated summary columns.

Q98. Female Representation in Department Leadership⌗

[!question] Problem Statement Calculate the historical percentage of female managers for each department.

[!tip] Concepts Tested Conditional Aggregation on dept_manager

[!success]- Reveal SQL Solution

SELECT d.dept_no, d.dept_name, 
       COUNT(*) AS total_managers_history, 
       SUM(CASE WHEN e.gender = 'F' THEN 1 ELSE 0 END) AS female_managers, 
       ROUND(SUM(CASE WHEN e.gender = 'F' THEN 1 ELSE 0 END) * 100.0 / COUNT(*), 2) AS female_leadership_pct 
FROM departments d 
JOIN dept_manager dm ON d.dept_no = dm.dept_no 
JOIN employees e ON dm.emp_no = e.emp_no 
GROUP BY d.dept_no, d.dept_name 
ORDER BY female_leadership_pct DESC;

Explanation: Analyzes gender representation in departmental leadership appointments.

Q99. Year-over-Year (YoY) Total Payroll Growth⌗

[!question] Problem Statement Calculate the Year-over-Year (YoY) percentage growth in total annual payroll expenditure.

[!tip] Concepts Tested LAG(), Window Function on Aggregated Annual Data

[!success]- Reveal SQL Solution

WITH AnnualPayroll AS (
    SELECT YEAR(from_date) AS payroll_year, 
           SUM(salary) AS total_payroll 
    FROM salaries 
    WHERE YEAR(from_date) BETWEEN 1986 AND 2000 
    GROUP BY payroll_year
),
PayrollGrowth AS (
    SELECT payroll_year, 
           total_payroll, 
           LAG(total_payroll, 1) OVER (ORDER BY payroll_year ASC) AS prior_year_payroll 
    FROM AnnualPayroll
)
SELECT payroll_year, 
       total_payroll, 
       prior_year_payroll, 
       ROUND(((total_payroll - prior_year_payroll) / prior_year_payroll) * 100, 2) AS yoy_growth_percentage 
FROM PayrollGrowth 
WHERE prior_year_payroll IS NOT NULL 
ORDER BY payroll_year ASC;

Explanation: Computes consecutive annual financial deltas and YoY percentage expansion.

Q100. Detect Overlapping Department Tenures⌗

[!question] Problem Statement Write an audit query to detect any employee who has simultaneous overlapping from_date and to_date intervals in dept_emp.

[!tip] Concepts Tested Self-Join Interval Intersection

[!success]- Reveal SQL Solution

SELECT de1.emp_no, 
       de1.dept_no AS dept_1, 
       de1.from_date AS start_1, 
       de1.to_date AS end_1, 
       de2.dept_no AS dept_2, 
       de2.from_date AS start_2, 
       de2.to_date AS end_2 
FROM dept_emp de1 
JOIN dept_emp de2 
  ON de1.emp_no = de2.emp_no 
 AND de1.from_date < de2.from_date 
WHERE de1.to_date > de2.from_date 
LIMIT 20;

Explanation: Essential data engineering and QA query that flags conflicting temporal assignments (start_1 < start_2 AND end_1 > start_2).

100 Must-Know MySQL Interview Questions & Solutions (Employees Dataset)

Table of Contents

100 MySQL Practice Questions & Answers⌗

🛠️ Schema Relationship Cheat Sheet⌗

🟢 Part 1: Basic SELECT, Filtering & Sorting⌗

Q1. Employees Hired After 1990⌗

Q2. All Distinct Historical Job Titles⌗

Q3. Pattern Matching on First and Last Names⌗

Q4. Female Employees Hired in Late 80s⌗

Q5. Substring Search in First Names⌗

Q6. Top 5 Highest Salaries Ever Recorded⌗

Q7. Long Department Names⌗

Q8. Active Employees Outside Department d005⌗

Q9. Current Salaries in 60k to 70k Range⌗

Q10. Employees Born in February⌗

Q11. Engineering and Management Titles⌗

Q12. Round Number Salaries⌗

Q13. The 10 Oldest Employees⌗

Q14. Currently Active Department Managers⌗

Q15. Equal Length First and Last Names⌗

🟢 Part 2: String & Date Functions⌗

Q16. Formatted Full Name (LASTNAME, Firstname)⌗

Q17. Current Employee Age⌗

Q18. Hire Age in Years⌗

Q19. Employees Hired on Mondays⌗

Q20. Custom Date Formatting⌗

Q21. Employees with Over 30 Years of Service⌗

Q22. Generate Corporate Email Addresses⌗

Q23. First Names with Odd Character Counts⌗

Q24. Birth Decade Distribution⌗

Q25. First and Last Day of Hire Month⌗

Q26. Masking Employee IDs⌗

Q27. Job Title Tenure in Days⌗

Q28. Shared Birthday Analysis⌗

Q29. Employees Hired on Their Birthday⌗

Q30. Sanitizing Department Names⌗

🟢 Part 3: Aggregations, GROUP BY & HAVING⌗

Q31. Gender Headcount and Percentage⌗

Q32. Hires Per Calendar Year⌗

Q33. Current Overall Salary Statistics⌗

Q34. Headcount by Active Job Title⌗

Q35. Departments with Over 15,000 Active Employees⌗

Q36. Average Current Salary by Department⌗

Q37. Titles with Average Salary Above $75,000⌗

Q38. Employees with the Most Salary Raises⌗

Q39. Department Salary Spread (> $50,000)⌗

Q40. High Volume Hiring Years (> 1,000 Hires)⌗

Q41. Total Active Payroll by Department⌗

Q42. Employees with 3 or More Distinct Titles⌗

Q43. Average Starting Salary by Birth Decade⌗

Q44. Department with the Highest Turnover of Managers⌗

Q45. Department & Gender Subtotals (WITH ROLLUP)⌗

🟢 Part 4: Relational Joins Mastery⌗

Q46. Current Employee Title and Salary⌗

Q47. Employees and Their Current Department⌗

Q48. All Current Department Managers⌗

Q49. Employees Alongside Their Current Manager⌗

Q50. Departments Without an Active Manager⌗

Q51. Employees Who Changed Departments⌗

Q52. Senior Engineers Earning Less Than Average Engineers⌗

Q53. Longest-Serving Manager per Department⌗

Q54. Employees Earning More Than Their Current Manager⌗

Q55. Empty Departments Check⌗

Q56. Staff with Same Title as Their Department Manager⌗

Q57. Department Peers Hired on the Same Day⌗

Q58. Transfers from Marketing (d001) to Finance (d002)⌗

Q59. Starting Salary for Each Title Held⌗

Q60. Unrepresented Title and Department Combinations⌗

🟢 Part 5: Subqueries, CTEs & Set Operations⌗

Q61. Single Highest Current Earner (No LIMIT)⌗

Q62. Employees Earning Above Company Average⌗

Q63. Departments with High Earners (> $100k) Using EXISTS⌗

Q64. Employees Without a Pay Raise (NOT EXISTS)⌗

Q65. Employees Earning Above Their Department Average⌗

Q66. Second Highest Current Salary⌗

Q67. The Nth Highest Salary (Generic Pattern)⌗

Q68. Employees Holding Both Senior Engineer and Technique Leader⌗

Q69. Initial Salary Higher Than Department Average Initial Salary⌗

Q70. Managers and $120k+ Earners (UNION)⌗

Q71. Employees in the Highest-Paying Department⌗