group by：通过某些字段进行分组
emp表：
+-------+--------+-----------+------+------------+---------+---------+--------+ | empno | ename | job | mgr | hiredate | sal | comm | deptno | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1987-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1987-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1987-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-01-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1987-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
例1：找出每个工作岗位的最高薪水
mysql> select max(sal) from emp group by job; +----------+ | max(sal) | +----------+ | 3000.00 | | 1300.00 | | 2975.00 | | 5000.00 | | 1600.00 | +----------+
***重点注意：
1.若一条DQL语句中有group by子句，那么select关键字后面只能跟参与分组的字段和分组函数
在MySQL中，语法比较松散，可以把其他的字段编写进去，但是查询结果会报错；
在Oracle中，语法比较严谨，如果输入其他子句会报错
2.group by后面可以跟多个字段，联合起来分组
例2：找出不同部门中不同工作岗位的最高薪水
mysql> select deptno,job,max(sal) from emp group by deptno,job; +--------+-----------+----------+ | deptno | job | max(sal) | +--------+-----------+----------+ | 10 | CLERK | 1300.00 | | 10 | MANAGER | 2450.00 | | 10 | PRESIDENT | 5000.00 | | 20 | ANALYST | 3000.00 | | 20 | CLERK | 1100.00 | | 20 | MANAGER | 2975.00 | | 30 | CLERK | 950.00 | | 30 | MANAGER | 2850.00 | | 30 | SALESMAN | 1600.00 | +--------+-----------+----------+
3.where子句后面为什么不能出现分组函数？：因为where子句的功能是在group by之前完成数据过滤，还没分组不能调用分组函数
having：分组之后进行过滤，一般能在where过滤的尽量在where过滤，避免无效数据参与分组
例3：找出每个岗位的平均薪水，要求显示平均薪水大于1500
mysql> select job,avg(sal) from emp group by job having avg(sal)>1500; +-----------+-------------+ | job | avg(sal) | +-----------+-------------+ | ANALYST | 3000.000000 | | MANAGER | 2758.333333 | | PRESIDENT | 5000.000000 | +-----------+-------------+
