select * from T_A a (nolock) where a.xh not in (select xh from T_B)
方法1：效果甚微
select * from T_A a (nolock) where not exists (select xh from T_B where xh=a.xh)
方法2：效果比想象的要好。
select * from T_A a (nolock) left join T_B b on b.xh=a.xh where b.xh is null
