APPLY Operator - CROSS APPLY and OUTER APPLY

CROSS APPLY: Invokes table-valued function/subquery per row; like INNER JOIN—excludes rows where function returns nothing. OUTER APPLY: Like LEFT JOIN—keeps all left rows, NULL where function returns nothing. Use: Correlated subqueries, JSON parsing, string splitting. PostgreSQL: LATERAL JOIN (CROSS JOIN LATERAL, LEFT JOIN LATERAL). Performance: Can be expensive—row-by-row. Prefer set-based when possible. Best practice: Use when row-by-row needed; avoid for large tables without indexing.