What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

Both UNION and UNION ALL concatenate the result of two different SQLs. They differ in the way they handle duplicates.

• UNION performs a DISTINCT on the result set, eliminating any duplicate rows.

• UNION ALL does not remove duplicates, and it therefore faster than UNION.

Note: While using this commands all selected columns need to be of the same data type.

Example: If we have two tables, 1) Employee and 2) Customer

1. Employee table data:

2. Customer table data:

3. UNION Example (It removes all duplicate records):

4. UNION ALL Example (It just concatenate records, not eliminate duplicates, so it is faster than UNION):