What is the difference between UNION and UNION ALL?

UNION: Combines results from two queries and removes duplicate rows

SELECT employee_id FROM employees WHERE department_id = 10
UNION
SELECT employee_id FROM employees WHERE salary > 50000;

UNION ALL: Combines results from two queries and keeps all rows including duplicates

SELECT employee_id FROM employees WHERE department_id = 10
UNION ALL
SELECT employee_id FROM employees WHERE salary > 50000;

Key Differences:

• UNION performs duplicate elimination (slower)
• UNION ALL returns all rows (faster)
• UNION sorts the result set to eliminate duplicates
• Use UNION ALL when you know there are no duplicates or duplicates are acceptable