When you are
performing a set operation such as a UNION, you need to abide by certain rules.
The types returned from the two sets must be compatible. There cannot be an
ORDER BY in either of the queries. If you do have different column names in the
queries, the result will get the names from the first set.
The UNION
operator implicitly does a DISTINCT on the results. That takes up some extra
time. If you don’t mind duplicates in the data, you can instead use the UNION
ALL operator that allows duplicates.
The INTERSECT operator returns items that are in
common between the two sides. This will also implicitly perform a DISTINCT and
not return any duplicates. The EXCCEPT operator returns a difference. It gives
you items in the first query that do not appear in the second query results.
