SQL for testers
SQL for QA is a tool to verify test data, prepare it, and confirm bugs at the backend level. The interview minimum:
- SELECT ... FROM ... WHERE — filtering
- ORDER BY, LIMIT — sorting and limiting
- COUNT, SUM, AVG + GROUP BY — aggregation
- JOIN (INNER/LEFT) — combining tables
- INSERT / UPDATE / DELETE — preparing test data (test environments only!)
INNER vs LEFT JOIN is the most popular SQL interview question: INNER — only rows matching in both tables; LEFT — all rows of the left table + matches from the right (NULL where none).
📚 Sources and documentation
- PostgreSQL: Querying a tableofficialpostgresql.org
- MySQL: SELECT statementofficialdev.mysql.com