Top 50 SQL Interview Questions — Crack Your Next Interview

1. What is the difference between WHERE and HAVING?

WHERE filters rows before GROUP BY. HAVING filters groups after GROUP BY.

SELECT city, COUNT(*) FROM students WHERE age > 18 GROUP BY city HAVING COUNT(*) > 5;

2. What is a PRIMARY KEY?

A column that uniquely identifies each row. Cannot be NULL or duplicate.

3. What is the difference between DELETE and TRUNCATE?

DELETE: Can use WHERE, logged, can rollback. TRUNCATE: Removes all rows, faster, cannot rollback.

11. What is a SELF JOIN?

A table joined with itself. Used when comparing rows in the same table.

SELECT e.name, m.name FROM employees e JOIN employees m ON e.manager_id = m.id;

15. What are Window Functions?

Functions that perform calculations across rows related to the current row. Examples: ROW_NUMBER(), RANK(), SUM() OVER().

35. What is query optimization?

Techniques to make SQL queries faster: indexing, avoiding SELECT *, using EXPLAIN, partitioning, query rewriting.