The difference between union and union all in mysql

The difference between UNION and UNION ALL in MySQL is that UNION eliminates duplicate rows, while UNION ALL retains all rows. UNION sorts the result set implicitly, but UNION ALL does not.

The difference between UNION and UNION ALL in MySQL

In MySQL, both UNION and UNION ALL are used Operator that combines row data from different query result sets. However, there are key differences in how the results are processed.

UNION

• Eliminate duplicate rows: The UNION operator eliminates duplicate rows from different query result sets. It keeps only unique rows in the result set.
• Implicit sorting: The UNION operator will implicitly sort the result set. It will sort the results based on how the data in the first query's result set is sorted.

UNION ALL

• Keep all rows: The UNION ALL operator keeps all rows from different query result sets, including duplicate rows. It does not eliminate duplicates.
• No implicit sorting: The UNION ALL operator does not implicitly sort the result set. The order of the result set will depend on the order in which the query is executed.

Usage scenarios

• Use UNION when you need to merge the results of different queries and eliminate duplicate rows.
• Use UNION ALL when you need to combine the results of different queries and retain all rows, including duplicate rows.

Example

Suppose there are two tables: table1 and table2, they both have a table named Column of name.

The following query uses UNION to merge the rows in the two tables and eliminate duplicate names:

SELECT name FROM table1
UNION
SELECT name FROM table2;

The following query uses UNION ALL to merge the rows in the two tables and retain duplicate names Name:

SELECT name FROM table1
UNION ALL
SELECT name FROM table2;

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