mysql delete前有沒有必要select一下

mysql在執(zhí)行delete操作前有沒有必要先select判斷下。

A:

select Amount into mAmount from list where uid = 12;
if mAmount = 1
then
    delete from list where uid = 12;
else
    update list set Amount = Amount - 1 where uid = 12;
end if;

B

delete from list where uid = 12 and Amount - 1  = 0;
if ROW_COUNT() <> 1
then
    update list set Amount = Amount - 1 where uid = 12;
end if;

a和b在高并發(fā)下哪個效果好點。