第六章分组函数=聚合函数 sum avg min max count个数行数

SELECT  AVG(sal), MAX(sal),
MIN(sal), SUM(sal)
FROM  emp
WHERE  job LIKE 'SALES%';

SELECT  MIN(hiredate), MAX(hiredate)FROM  emp;

SELECT  COUNT(*)FROM  emp
WHERE  deptno = 30;
 （*是列名最好是主键没有重复）
否则
SELECT count(DISTINCT deptno) FROM dept;

SELECT AVG(sal) as 销售的平均工资,sum(sal) as 销售的总工资 from emp where job like 'sales%'```