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MSDN HELP
The UNION clause combines the results from two or more SQL SELECT statements into a single result set containing rows from all the queries in the UNION operation.

Note
When one of the columns is of Memo, General, or Blob type, performing unions of differing column types is not allowed.


For the complete syntax, see SELECT - SQL Command.

The detailed syntax for the UNION clause is as follows:


[UNION [ALL] SELECTCommand]



Parameters
[UNION [ALL] SELECTCommand]
Specifies another SELECT statement. By default, UNION eliminates duplicate rows from the combined result set.

Note
The ALL keyword includes duplicate rows in the combined result set.