What's the difference between UNION and UNION ALL?

beginner

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UNION: Combines result sets and removes duplicate rows, slower due to duplicate elimination
UNION ALL: Combines result sets keeping all rows including duplicates, faster execution
Both require same number of columns with compatible data types in the same order.

SELECT name FROM employees_2023
UNION                    -- Removes duplicates
SELECT name FROM employees_2024;
SELECT name FROM employees_2023
UNION ALL               -- Keeps all rows
SELECT name FROM employees_2024;

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