子查询不要出现太多,大多数能够使用外连接来替代select *, (select z1 from T1 where z2=T2.Id) z1, (select z2 from T1 where z2=T2.Id) z2, (select z3 from T1 where z2=T2.Id) z3, from T2这段语句肯定不是优化的,会有性能问题的,可以有如下的方式来改写select *, T1.Z1, T1.Z2, T1.Z3from T2 left outer join T1 on T1.Z2 = T2.ID
from T2,T1
where T1.z2='1'
from T1,T2
where T1.z2='1'
(select z1 from T1 where z2=T2.Id) z1,
(select z2 from T1 where z2=T2.Id) z2,
(select z3 from T1 where z2=T2.Id) z3,
from T2这段语句肯定不是优化的,会有性能问题的,可以有如下的方式来改写select *, T1.Z1, T1.Z2, T1.Z3from T2 left outer join T1 on T1.Z2 = T2.ID