Practice SQL interview questions covering DDL/DML, joins, subqueries, indexing, query optimization, transactions, window functions, and stored procedures.
SQL knowledge is tested in almost every data-adjacent interview — backend, data engineering, analytics, and even many full-stack roles. The difference between a candidate who knows SQL syntax and one who can write optimised, production-quality queries is usually visible within the first two questions.
Command all join types precisely: INNER (only matching rows), LEFT/RIGHT OUTER (include non-matching from one side), FULL OUTER (include all non-matching), CROSS (cartesian product), and SELF join. Master subqueries and CTEs — know when a CTE is preferable to a derived table (readability, recursion), and when a correlated subquery will cause performance problems that a JOIN would avoid. Window functions are the most commonly under-prepared topic: ROW_NUMBER(), RANK(), DENSE_RANK(), LEAD(), LAG(), SUM() OVER(), and PARTITION BY semantics — these distinguish candidates who understand set-based SQL thinking.
Query optimisation requires understanding index selection, when the planner uses an index scan vs a sequential scan, the cost of JOINs on un-indexed foreign keys, and how GROUP BY and ORDER BY interact with indexes. Transaction isolation levels and ACID properties round out the must-know areas for data-heavy roles. Use the Top 50 SQL Interview Questions to practise complete, production-quality SQL answers.
