UNION removes duplicate records (where all columns in the results are the same),
UNION ALL does not.
There is a performance hit when using
UNION instead of
UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
To identify duplicates, records must be comparable types as well as compatible types. This will depend on the SQL system. For example the system may truncate all long text fields to make short text fields for comparison (MS Jet), or may refuse to compare binary fields (ORACLE)
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar
+-----+ | bar | +-----+ | foo | +-----+ 1 row in set (0.00 sec)
UNION ALL example:
SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar
+-----+ | bar | +-----+ | foo | | foo | +-----+ 2 rows in set (0.00 sec)