The SQL UNION is used to combine the results of two or more SELECT SQL statements into a single result. All the statements concatenated with UNION must have the same structure. This means that they need to have the same number of columns, and corresponding columns must have the same or compatible data types (implicitly convertible to the same type or explicitly converted to the same type). The columns in each SELECT statement must be in exactly the same order too.

The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.