The two queries must result in the same number of columns and compatible data types in order to unite.
UNION can be useful in data warehouse applications where tables are not perfectly normalized.
[2] A simple example would be a database having tables sales2005 and sales2006 that have identical structures but are separated because of performance considerations.
For purposes of duplicate removal the INTERSECT operator does not distinguish between NULLs.
The INTERSECT operator removes duplicate rows from the final result set.
The following example INTERSECT query returns all rows from the Orders table where Quantity is between 50 and 100.
For purposes of row elimination and duplicate removal, the EXCEPT operator does not distinguish between NULLs.