--方法1：Update Table1 set c = (select c from Table2 where a = Table1.a) where c is null --方法2：update Aset newqiantity=B.qiantityfrom A,Bwhere A.bnum=B.bnum --方法3：update (select A.bnum ,A.newqiantity,B.qiantity from A left join B on A.bnum=B.bnum) AS Cset C.newqiantity = C.qiantitywhere C.bnum =XX
