[ MySQL ] – 幫查詢結果加上 流水號

最近有個需求要在mysql查詢時自動加上流水號，不可以透過php來做這件事><
[php]
SELECT (@sno := @sno+1) as sno,a,b FROM `table`,(select @sno:=0) as tmp WHERE 1 ORDER BY a desc
[php]
顯示結果
sno,a,b
1,x1,z1
2,x44,z45
3,x55,z76
4,x33,z99