聚合函数

Posted 2023-02-25 嫚嫚_

# 第八章 聚合函数
-- 1：常见的聚合函数
-- 1.1 AVG/SUM  只用于数值的字符
SELECT AVG(salary),SUM(salary),AVG(salary)*107
FROM employees;


-- 1.2 max /min

SELECT MAX(salary),MIN(salary)
FROM employees;

SELECT MAX(last_name)
FROM employees;
-- 1.3 count  数个数 不包含空值
SELECT COUNT(employee_id),COUNT(salary),COUNT(1)
FROM employees;

-- 2：group by 的使用 
# 需求查询各个部门的平均工资
SELECT department_id,AVG(salary)
FROM employees
GROUP BY department_id


SELECT job_id,AVG(salary)
FROM employees
GROUP BY job_id;

# 需求：查询各个department_id,job_id的工资
SELECT  department_id dept_id, job_id, SUM(salary)
FROM   employees
GROUP BY department_id, job_id ;
-- 3：having的使用(作用：用来过滤数据)
# 要求一：如果过滤条件使用了聚合函数
# 要求二：having必须使用在group by 后面
# 要求三：开发中，我们使用having 的前提是SQL 中使用了GROUP BY
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary)>10000;

# 练习 查询部门id 为 10，20，30，40这四个部门工资高10000的信息
-- 方式一(推荐)
SELECT department_id,MAX(salary)
FROM employees
WHERE department_id IN(10,20,30,40)
GROUP BY department_id
HAVING MAX(salary)>10000;
-- 方式二
SELECT department_id,MAX(salary)
FROM employees

GROUP BY department_id
HAVING MAX(salary)>10000 AND  department_id IN(10,20,30,40);

-- 结论 当过滤条件中有聚合函数时，过滤条件必须声明在HAVING中
-- 结论 当过滤条件中没有聚合函数时，过滤条件必须声明在where中

-- SELECT ... FROM ... WHERE ... GROUP BY ... HAVING ... ORDER BY ... LIMIT...