oracle where与having

where与having可以过滤，一般来说尽量使用where ，但是如果过滤条件中有组函数，只能使用having

 SQL> select deptno,avg(sal)
   2    from emp
   3    where deptno=10
   4    group by deptno;

 DEPTNO   AVG(SAL)
 ------ ----------
     10       2950

 SQL>
 SQL> select deptno,avg(sal)
   2    from emp
   3    where avg(sal)>2000
   4    group by deptno;

 select deptno,avg(sal)
   from emp
   where avg(sal)>2000
   group by deptno

 ORA-00934: 此处不允许使用分组函数

 SQL>
 SQL> select deptno,avg(sal)
   2    from emp
   3    group by deptno
   4    having avg(sal)>2000;

 DEPTNO   AVG(SAL)
 ------ ----------
     20 2258.33333
     10       2950