需要分组的字段要包含到查询字段中
exp:select deptno from emp group by deptno;
与count函数连用:
exp:select deptno,count(deptno) count from emp group by deptno;
select deptno,count(deptno) from emp;
错误:"not a single-group group function"
解释:聚合函数与其他字段一起作为查询字段时需要进行分组,并且只能包含分组字段,;
exp:select count(deptno) from emp group by deptno;
select deptno,count(deptno) from emp group by deptno;
select sal,count(deptno) from emp group by deptno;(错误)