SQL Interview Prep Questions

Create a SQL interview preparation set for a [JUNIOR / MID / SENIOR] data role.

Database: [POSTGRESQL / MYSQL / SQL SERVER]

Provide 20 questions in increasing difficulty:

**Easy (5 questions)**: Basic SELECT, WHERE, JOIN, GROUP BY, ORDER BY
**Medium (8 questions)**: Subqueries, window functions, CASE statements, CTEs, self-joins
**Hard (5 questions)**: Complex aggregations, recursive CTEs, pivoting, performance optimization
**Brain Teasers (2 questions)**: Tricky logic problems solved with SQL

For each question:
- Business scenario and question
- Sample table structure with data
- Expected output
- Complete SQL solution with explanation
- Common mistakes candidates make
- Follow-up question the interviewer might ask

Create a SQL interview preparation set for a [JUNIOR / MID / SENIOR] data role.

Database: [POSTGRESQL / MYSQL / SQL SERVER]

Provide 20 questions in increasing difficulty:

**Easy (5 questions)**: Basic SELECT, WHERE, JOIN, GROUP BY, ORDER BY
**Medium (8 questions)**: Subqueries, window functions, CASE statements, CTEs, self-joins
**Hard (5 questions)**: Complex aggregations, recursive CTEs, pivoting, performance optimization
**Brain Teasers (2 questions)**: Tricky logic problems solved with SQL

For each question:
- Business scenario and question
- Sample table structure with data
- Expected output
- Complete SQL solution with explanation
- Common mistakes candidates make
- Follow-up question the interviewer might ask