UNION ALL clauses combine the result-set of two or more identically structured SELECT statements into a single result / table.
Both the column count and column types for each query have to match in order for a
UNION ALL to work.
The difference between a
UNION and a
UNION ALL query is that the
UNION clause will remove any duplicate rows in the result where the
UNION ALL will not.
This distinct removal of records can significantly slow queries even if there are no distinct rows to be removed because of this if you know there wont be any duplicates (or don't care) always default to
UNION ALL for a more optimised query.