MySQL Note-union

Union syntax

Select ...

Union [all | distinct]

Selct ...

1. The union is used to combine results from multiple SELECT statements in a single result set.
2. the list of two queries must be the same , otherwise the "The used SELECT statements has a different number of columns"
3. Multiple SELECT statements can be removed with different field names, without error, but when the final result set is displayed, the field name of the first SELECT statement will prevail.
4. When you remove the same data from multiple SELECT statements, union merges the same columns, and it must be explained that the so-called same means that all the fields are identical in content
5. If you do not want to remove the duplicate rows, use the UNION ALL
6. If there is an order by,limit in the clause, enclose it in parentheses (). After all clauses are recommended, the result set of the final merge is sorted or filtered.
7. In clauses, the order by needs to be used with limit to make sense. If not used with limit, it will be removed by the parser optimization analysis.

MySQL Note-union