In SQL, UNION and UNION ALL are used to combine the results of two or more SELECT statements. However, they have some key differences in how they handle duplicate rows.

UNION

• Definition: The UNION operator combines the result sets of two or more SELECT statements and removes duplicate rows from the final result set.
• Usage: It is used when you want to ensure that the result set contains only unique records.
• Performance: Because it removes duplicates, UNION may be slower than UNION ALL, especially with large datasets, as it requires additional processing to identify and eliminate duplicates.

Example:

sql
1SELECT column1 FROM table1
2UNION
3SELECT column1 FROM table2;

In this example, the result will include unique values from both table1 and table2.

UNION ALL

• Definition: The UNION ALL operator also combines the result sets of two or more SELECT statements, but it includes all rows, including duplicates.
• Usage: It is used when you want to include all records from the combined result sets, regardless of whether they are duplicates.
• Performance: UNION ALL is generally faster than UNION because it does not require the additional step of removing duplicates.

Example:

sql
1SELECT column1 FROM table1
2UNION ALL
3SELECT column1 FROM table2;

In this example, the result will include all values from both table1 and table2, including any duplicates.

Summary of Differences

In summary, choose UNION when you need a distinct set of results and UNION ALL when you want to include all results, including duplicates.