update a
2 set
3 a.col1=(select col3 from a,b where a.col1=b.col1)
4 where a.col1=(select a.col1 from a,b where a.col1=b.col1)
5 ;
改成
update a
2 set
3 a.col1=(select col3 from a,b where a.col1=b.col1)
4 where exists (select col3 from a,b where a.col1=b.col1);
会快一些