SQL Interview Questions

GROUP BY with HAVING is bread-and-butter SQL. If a candidate can't write this without help, they'll struggle with any reporting or analytics work.

JOIN type confusion causes silent data loss or duplication in queries. A candidate who can explain when LEFT JOIN produces NULLs understands their data, not just the syntax.

Indexes are the single biggest performance lever in SQL. A developer who thinks indexes are free will create write bottlenecks, and one who doesn't use them will create read bottlenecks.

Removing the LIMIT/OFFSET crutch forces candidates to use subqueries or window functions. It's a quick test of SQL problem-solving beyond basic patterns.

Using WHERE with aggregate functions is one of the most common SQL mistakes. This catches it early and shows whether the candidate understands query execution order.

Running untested queries against production is how data gets deleted. This shows whether the candidate has safe habits and knows about EXPLAIN plans.

NULL handling trips up developers at every level. Writing WHERE x = NULL instead of IS NULL is a bug that returns zero rows and can silently break reports.