The UNION operator in MySQL is used to combine the result sets of two or more SELECT statements into a single result set. The result set obtained from the UNION operator will contain all the distinct rows returned by the SELECT statements in the union.
The syntax for using the UNION operator is as follows:
SELECT column1, column2, ... FROM table1 UNION SELECT column1, column2, ... FROM table2;
In this syntax, the
SELECT statements must have the same number of columns, and the data types of the corresponding columns must be compatible. The column names in the result set are determined by the column names specified in the first SELECT statement.
By default, the UNION operator removes duplicate rows from the result set. However, if you want to include duplicates, you can use the UNION ALL operator, like this:
SELECT column1, column2, ... FROM table1 UNION ALL SELECT column1, column2, ... FROM table2;
Using UNION and UNION ALL can be useful when you want to combine data from two or more tables with similar structures, such as when you need to merge the results of two queries that have the same columns but different filtering conditions or sorting orders.