25 SQL Interview Questions: Queries, Joins & Window Functions

SQL Interview Questions 2026: From JOINs to Window Functions

SQL is tested in 100% of data science interviews, 80% of backend interviews, and 60% of full-stack interviews. The questions range from basic SELECT/JOIN to advanced window functions, CTEs, and query optimization. According to HackerRank, SQL is the #2 most tested skill after algorithms.

The three SQL skills that separate junior from senior: 1) Window functions (ROW_NUMBER, RANK, LAG, LEAD), 2) CTEs for readable complex queries, 3) Query optimization using EXPLAIN ANALYZE and indexing strategies.

Essential SQL Topics

• JOINs — INNER, LEFT, RIGHT, FULL, CROSS, self-joins
• Aggregation — GROUP BY, HAVING, COUNT, SUM, AVG with conditions
• Window Functions — OVER, PARTITION BY, ROW_NUMBER, RANK, running totals
• CTEs — WITH clause for readable, reusable subqueries
• Optimization — Indexes, EXPLAIN plans, query rewriting

Related: data engineering questions, data science tips.