The Union of MySQL

Write about the use of Union today and something to be aware of.

Union: The union means to merge two or more query results together.

Requirements: Two queries must have the same number of columns

Recommendation: The type of column can be different, but the recommended query for each column, want the corresponding type to be the same

Data that can come from multiple tables: The column names taken out by multiple SQL statements can be inconsistent, with the column name of the first SQL statement as a standard.

If the rows taken in different statements are identical (in this case, the values for each column are the same), then union merges the same rows and eventually retains only one row. It is also possible to understand that union removes duplicate rows.

If you do not want to remove duplicate rows, you can use union ALL.

If there is an order by,limit in the clause, enclose it in parentheses (). After all clauses are recommended, the result of the final merge is sorted or filtered.

such as: (SELECT * from a ORDER by ID) union (SELECT * from B order ID);

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.

The Union of MySQL