MySQL Union vs Union All

The MySQL has both UNION and UNION ALL are used to combine the result sets of two or more SELECT statements into a single result set. However, they differ in how they handle duplicate rows in the combined result:

1. UNION:

• The UNION operator removes duplicate rows from the combined result set. If the same row appears in multiple result sets, it will appear only once in the final result.
• It is slower than UNION ALL because it requires the database to perform a distinct operation to eliminate duplicates.
• Use UNION when you want to retrieve distinct values from the combined result. Example:

   SELECT name FROM table1
   UNION
   SELECT name FROM table2;

2. UNION ALL:

• The UNION ALL operator combines all rows from the result sets, including duplicate rows. If a row appears in multiple result sets, it will appear multiple times in the final result.
• It is faster than UNION because it does not require the removal of duplicates.
• Use UNION ALL when you want to retrieve all rows from the combined result, including duplicates. Example:

   SELECT name FROM table1
   UNION ALL
   SELECT name FROM table2;

Here are some considerations when choosing between UNION and UNION ALL:

• Use UNION when you want to eliminate duplicate rows and retrieve only distinct values.
• Use UNION ALL when you want to combine all rows from the result sets, even if there are duplicates. This can be more efficient if you know there are no duplicates or if you want to preserve duplicate rows for a specific reason.
• In terms of performance, UNION ALL is generally faster than UNION because it doesn’t require the database to perform the additional step of eliminating duplicates.

The choice between UNION and UNION ALL depends on the specific requirements of your query and whether you need distinct values or not.