SELECT 1,22,44 UNION SELECT 2,33,55

http://i.stack.imgur.com/rF9SA.png

SELECT 1,22,44 UNION SELECT 2,33,55 UNION SELECT 2,33,55

The result is the same as above.

use UNION ALL

when

SELECT 1,22,44 UNION SELECT 2,33,55 UNION ALL SELECT 2,33,55

http://i.stack.imgur.com/tD1Rz.png