FROM中子查询

 select e.ename,subq.loc
     from emp e,
      (select deptno,loc from dept where loc in('NEW YORK','DALLAS') ) subq
   where e.deptno=subq.deptno;
ENAME      LOC
---------- -------------
CLARK      NEW YORK
KING       NEW YORK
MILLER     NEW YORK
……
SQL> select ename,job,sal,rownum
    from (select ename,job,sal from emp order by sal)
     where rownum<=3;
ENAME      JOB              SAL     ROWNUM
---------- --------- ---------- ----------
SMITH      CLERK            800          1
JAMES      CLERK            950          2
ADAMS      CLERK           1100        3

《ps》 主要解决主查询中读取子查询中的列信息，类似  select aaa,bbb from A, B;