exists,in,not exists,not in

1 exists && in
   

  --EXISTS的执行流程        
  select * from t1 where exists ( select null from t2 where y = x 
  --可以理解为:
    for x in ( select * from t1 )
     loop
        if ( exists ( select null from t2 where y = x.x )
        then 
           OUTPUT THE RECORD
        end if
     end loop
    [b][size=small]1.2  in[/size][/b]
  -----------------------------------------------------------
  --IN的执行流程
  select * from T1 where x in ( select y from T2 )
  --可以理解为:
  select * from t1, ( select distinct y from t2 ) t2 where t1.x = t2.y;

2 not in &&  not exists
    2.1 not in
   1,例子：从A表中查出来一个集合，从B表中查出来一个集合，且A表中包含了B表中的集合，求A表中不包含B表的集合

select a.iid from A a where a.iid not in(select iid from B)  

    2.2 not exists