MySQL数据库：第六章：分组函数/聚合函数

SELECT SUM(salary),AVG(salary),MAX(salary),MIN(salary),COUNT(salary) FROM employees;

SELECT SUM(last_name),AVG(last_name),MAX(last_name),MIN(last_name),COUNT(last_name) FROM employees;
SELECT SUM(hiredate),AVG(hiredate),MAX(hiredate),MIN(hiredate),COUNT(hiredate) FROM employees;

五个分组函数都忽略null
SELECT SUM(commission_pct),AVG(commission_pct),MAX(commission_pct),MIN(commission_pct),COUNT(commission_pct) FROM employees;
SELECT AVG(commission_pct),SUM(commission_pct)/107,SUM(commission_pct)/35 FROM employees;

 count(distinct 字段名)
    SELECT COUNT(department_id) FROM employees;
    #案例：员工涉及到了几个部门
    SELECT COUNT(DISTINCT department_id) FROM employees;
    #5.count的实参形式
    SELECT COUNT(*) FROM employees;
    SELECT COUNT(1) FROM employees;
    #6.添加筛选条件
    SELECT COUNT(*)  FROM employees WHERE department_id = 30;
    #7.不是所有的字段都能和分组函数一同查询出来的
    SELECT COUNT(*) ,employee_id FROM employees;