I try to avoid UNION clauses in most SQL that I write. Historically, they have taken too much of a toll on performance. Now we have a speedier alternative to the UNION clause — UNION ALL.
In a nutshell, a UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
This difference is that UNION ALL will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.
UNION Definition:
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL Definition:
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
A great reference can be found in Pinal Dave’s blog.