UNION vs UNION ALL

UNION combines results and removes duplicates.

UNION ALL combines results and keeps duplicates.

SELECT city FROM customers
UNION
SELECT city FROM suppliers;

Duplicates are removed.

SELECT city FROM customers
UNION ALL
SELECT city FROM suppliers;

Duplicates remain.

Which is faster?

Usually UNION ALL, because it doesn't need to perform duplicate elimination.

Use:

• UNION → when duplicates must be removed
• UNION ALL → when you know duplicates are valid or don't care about them