select a.mc,a.zdkc,b.sl
from a join(select mc,sl=sum(sl) form b group by mc)b on a.mc=b.mc
where a.zdkc <0
from a join(select mc,sl=sum(sl) form b group by mc)b on a.mc=b.mc
where a.zdkc <0
a如何join到b上面去多读读zjcxc的语句。