Difference between SQL Union VS Union All.

UNION

1. Removes duplicate rows
2. Performs extra sorting/comparison → slower
3. Use when you need unique results only

UNION ALL

1. Keeps all rows (including duplicates)
2. No duplicate check → faster
3. Use when duplicates are acceptable or needed

Example:

SELECT Name FROM Students
UNION
SELECT Name FROM Teachers;
-- returns unique names only

SELECT Name FROM Students
UNION ALL
SELECT Name FROM Teachers;
-- returns all names including duplicates