UPDATE 表关联优化
优化前:
update table1 a set co1 =(select co2 from table2 b where b.co3=a.co3 )
where exists =(select 1 from table2 b where b.co3=a.co3 )
优化后:
update (select a.co1,b.co2 from sa.table1 a , sa.table2 b where b.co3=a.co3 ) set co1= co2
b.co3=a.co3必须为唯一