UNION is used to combine the result from many SELECT
statements into one result set.  UNION is available from MySQL 4.0.0
on.

Selected columns listed in corresponding positions of each SELECT
statement should have the same 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.) The column names used in
the first SELECT statement are used as the column names for the
results returned.
