SQL · Chapter 20 of 42

SQL UNION

UNION combines the results of two SELECTs into one, stacking them vertically. Columns must match in count and compatible types.

`UNION` removes duplicates. `UNION ALL` keeps them (and is faster).

UNION vs UNION ALL

UNION does a dedupe pass — slower. UNION ALL is faster and preferred when you know rows are distinct.

Example 1 (sql)

SELECT name FROM staff
UNION
SELECT name FROM interns;

Output

distinct names from both tables

UNION deduplicates.

Example 2 (sql)

SELECT id, 'staff' AS kind FROM staff
UNION ALL
SELECT id, 'intern' FROM interns;

Output

every row, tagged with source

UNION ALL keeps duplicates and preserves counts.

💡 Note: ORDER BY only appears at the END, after the final SELECT — it applies to the combined result.