Syntax:
SELECT ...
UNION [ALL | DISTINCT] SELECT ...
[UNION [ALL | DISTINCT] SELECT ...]

UNION is used to combine the result from multiple SELECT statements
into a single result set.

The column names from the first SELECT statement are used as the column
names for the results returned. Selected columns listed in
corresponding positions of each SELECT statement should have the same
data type. (For example, the first column selected by the first
statement should have the same type as the first column selected by the
other statements.)

URL: http://dev.mysql.com/doc/refman/5.1/en/union.html

