SQL interview essentials

Interview SQL tests JOINs, aggregates, NULL logic, indexing intuition, and transaction basics—not obscure vendor trivia. Explain your reasoning aloud while writing queries.

Classic patterns

-- Second highest salary (sketch)
SELECT MAX(salary) FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

-- Duplicate emails
SELECT email, COUNT(*) AS c
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

Practice: Solve on paper, then verify in SQLite.

Topics to rehearse

• INNER vs LEFT JOIN and orphan detection
• WHERE vs HAVING; GROUP BY rules
• NULL comparisons and COALESCE
• Indexes and EXPLAIN at a high level
• Normalization and when to denormalize
• SQL injection and parameterized queries

Communication tips

Clarify schema assumptions, start with simple SELECT, add filters, then optimize. Mention trade-offs (index write cost, pagination OFFSET vs keyset). Pair verbal SQL skills with app knowledge from Python or PHP tracks.

Important interview questions and answers

1. Q: Find duplicates interview?
   A: GROUP BY key HAVING COUNT(*) > 1.
2. Q: Index trade-off?
   A: Faster reads, slower writes and extra storage—index selective query columns.

Self-check

1. How find rows with duplicate email?
2. Explain LEFT JOIN + IS NULL pattern in one sentence.

Tip: Talk through JOIN type choice aloud—communication matters as much as syntax.