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.
This is how a simple UNION statement looks like:
SELECT Column1, Column2 FROM Table1
SELECT Column1, Column2 FROM Table2
The column names in the result of a UNION are always the same as the column names in the first SELECT statement in the UNION.
The UNION operator removes by default duplicate rows from the result set. You have the option to use the ALL keyword after the UNION keyword, which will force all rows including duplicates to be returned in your result set.