When should you use a subquery vs a JOIN?

JOINs are typically preferred: optimizers handle them well, easier to read, can return columns from both tables. Use subqueries when: checking existence (EXISTS), comparing against aggregates, or for clarity in complex logic. Correlated subqueries (reference outer query) execute per row - can be slow. Modern optimizers often transform subqueries to joins. EXISTS can outperform IN for large datasets. Always test with execution plans - performance varies by query and data.