**Performance:**

- NOT EXISTS often faster than NOT IN

- NOT IN with subquery can be slow

- Negation can prevent index usage

- Consider LEFT JOIN with NULL check for NOT EXISTS

NOT EXISTS often faster than NOT IN

Negation can prevent index usage

Filter NULL early in NOT IN subqueries

Consider LEFT JOIN for complex NOT EXISTS

Test query plans for optimization

NOT IN with NULL returns no rows

Forgetting NULL behavior in negation

Double negatives (hard to read)

Using NOT IN when NOT EXISTS better

Not testing with NULL data