http://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html
SELECT * FROM t1
ORDER BYkey_part1
,key_part2
,... ;
SELECT * FROM t1
WHEREkey_part1
=constant
ORDER BYkey_part2
;
SELECT * FROM t1
ORDER BYkey_part1
DESC,key_part2
DESC;
SELECT * FROM t1
WHEREkey_part1
=1
ORDER BYkey_part1
DESC,key_part2
DESC;