UNION and UNION ALL

UNION combines result sets from two queries vertically (stacking rows).

SELECT first_name FROM employees WHERE department_id = 1
UNION
SELECT first_name FROM employees WHERE salary > 100000;

UNION removes duplicate rows automatically. UNION ALL keeps all rows, including duplicates.

Rules for UNION:

• Both queries must have the same number of columns
• Columns must have compatible data types
• Column names come from the first query

Use UNION ALL when you know there are no duplicates - it's faster.