In SQL Server, both UNION and UNION ALL are used to combine the results of two or more SELECT statements. However, they have a key difference:
UNION:
Removes Duplicate Rows: It performs a DISTINCT operation on the result set, which means it eliminates any duplicate rows.
Use Case: When you need a combined result set without any duplicates.
UNION ALL:
Includes Duplicate Rows: It does not remove duplicates, so all rows from the combined SELECT statements are included.
Use Case: When you need all rows, including duplicates, which can be faster since it skips the DISTINCT operation.
