SQL & Databases Interview Prep
Test & Track Your Readiness

INNER, LEFT, RIGHT, FULL OUTER, SELF joins — and the fan-out trap that catches everyone

GROUP BY, HAVING, COUNT/SUM/AVG with NULLs, and conditional aggregation patterns

ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, running totals, and the LAST_VALUE frame gotcha

WITH clause, recursive CTEs for hierarchies, and the PostgreSQL optimization fence trap

Correlated vs uncorrelated subqueries, NOT IN with NULLs, EXISTS vs IN performance, LATERAL joins

B-tree internals, composite index left-prefix rule, covering indexes, functional indexes, write overhead

EXPLAIN ANALYZE, N+1 queries, cursor vs offset pagination, hash join vs nested loop

1NF–3NF, intentional denormalization, soft deletes with partial indexes, table partitioning

ACID, isolation levels, dirty/phantom reads, deadlock prevention, SELECT FOR UPDATE, optimistic locking

UNION vs UNION ALL performance, INTERSECT for overlap, EXCEPT for difference, ORDER BY rules

Three-valued logic, NULL = NULL trap, COALESCE, NULLIF, aggregate functions silently ignoring NULLs

DATE_TRUNC, index-hostile function wrapping, STRING_AGG, TIMESTAMPTZ vs TIMESTAMP timezone bugs