INTERSECT and EXCEPT

UNION stacks two results together. Two more set operators compare them instead:

• INTERSECT keeps rows that appear in both results
• EXCEPT keeps rows from the first result that aren't in the second

Who manages nobody? Take every employee id, then remove every id that appears as someone's manager:

SELECT id FROM employees
EXCEPT
SELECT manager_id FROM employees;

Which departments have someone earning over 130,000 and also someone hired since 2022?

SELECT department_id FROM employees WHERE salary > 130000
INTERSECT
SELECT department_id FROM employees WHERE hire_date >= '2022-01-01';

The rules are the same as UNION:

• Both queries need the same number of columns
• Duplicates are removed from the result
• ORDER BY goes once, at the very end

Oracle calls EXCEPT "MINUS", but it works the same way.