Set Operations Subquery
Set Operations Subquery is a SQL statement in the Set Operations category. Wrap a set operation in a subquery/derived table to apply additional filtering or joins. The syntax is SELECT * FROM (SELECT ... UNION SELECT ...) AS derived WHERE condition;. It returns filtered combined result. A typical example: SELECT * FROM ( SELECT id, name, salary, 'Current' AS status FROM employees UNION ALL SELECT id, name, salary, 'Former' FROM terminated_employees ) AS all_employees WHERE salary > 50000 ORDER BY salary DESC; -- Combined employee list filtered by salary A close relative is UNION, which combines results from two queries, removing duplicate rows. Each SELECT must have the same number of columns. A close relative is UNION ALL, which like UNION but does NOT remove duplicate rows. Faster because no dedup is needed. A close relative is INTERSECT, which returns rows that appear in BOTH query results. Supported in PostgreSQL, SQL Server, Oracle.