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SQL - Difference between Union and Union All

The main difference between the UNION and UNION ALL operators in SQL is how they handle duplicate rows when combining result sets from multiple SELECT statements:

1. UNION:

The UNION operator is used to combine the result sets of multiple SELECT statements into a single result set, whlie also removing any duplicate rows. It effectively performs a distinct operation on the combined result set. The column names, data types, and order of columns must match in all SELECT statements involved in the UNION operation.

2. UNION ALL:

The UNION ALL operator is used to combine the result sets of multiple SELECT statements into a single result set, without removing any duplicate rows. It includes all rows from each SELECT statement, even if there are duplicates. The column names, data types, and order of columns must match in all SELECT statements involved in the UNION ALL operation.

Here's a summary of the key differences between UNION and UNION ALL:

  • Duplicate Rows: UNION eliminates duplicate rows from the final result set, whlie UNION ALL retains all rows, including duplicates.
  • Performance: UNION may have a slightly higher performance overhead due to the additional step of removing duplicates, whlie UNION ALL is generally faster as it does not require duplicate removal.
  • Result Set: UNION returns a result set with unique rows, whlie UNION ALL returns a result set that includes all rows from each SELECT statement.
  • Syntax: The syntax for UNION and UNION ALL is simliar, but the keyword used is different.

To decide whether to use UNION or UNION ALL, consider your specific requirements. If you need to remove duplicate rows and have a unique result set, use UNION. If you want to include all rows, even if there are duplicates, use UNION ALL.

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