What is the difference between the Union and Union All operators in SQL?

• UNION combines the result sets of two or more SELECT queries, removing duplicate rows.
• UNION ALL also combines result sets, but does not remove duplicates, keeping all rows from all queries.

The key difference is in how duplicates are handled. Due to the need to sort and compare rows to remove duplicates, UNION is usually slower than UNION ALL, especially on large datasets.

Characteristic	UNION	UNION ALL
Duplicate handling	Removes duplicates	Keeps all rows
Performance	Can be slower	Usually faster
Sorting for duplicates	Required	Not required