1. 常见的几个聚合函数

1.1 AVG / SUM 只适用于数值类型的字段或变量

SELECT AVG(salary),SUM(salary),AVG(salary) * 107
FROM employees;
#如下的操作没有意义
SELECT SUM(last_name),AVG(last_name),SUM(hire_date)
FROM employees;

1.2 MAX / MIN :适用于数值类型、字符串类型、日期时间类型的字段或变量

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

1.3 COUNT

# ① 作用计算指定字段在查询结构中出现的个数不包含NULL值的
SELECT COUNT(employee_id),COUNT(salary),COUNT(2 * salary),COUNT(1),COUNT(2),COUNT(*)
FROM employees ;
SELECT *
FROM employees;
#如果计算表中有多少条记录如何实现
#方式1COUNT(*)
#方式2COUNT(1)
#方式3COUNT(具体字段) : 不一定对
#② 注意计算指定字段出现的个数时是不计算NULL值的。
SELECT COUNT(commission_pct)
FROM employees;
SELECT commission_pct
FROM employees
WHERE commission_pct IS NOT NULL;
#③ 公式AVG = SUM / COUNT
SELECT AVG(salary),SUM(salary)/COUNT(salary),
AVG(commission_pct),SUM(commission_pct)/COUNT(commission_pct),
SUM(commission_pct) / 107
FROM employees;
#需求查询公司中平均奖金率
#错误的
SELECT AVG(commission_pct)
FROM employees;
#正确的
SELECT SUM(commission_pct) / COUNT(IFNULL(commission_pct,0)),
AVG(IFNULL(commission_pct,0))
FROM employees;
# 如何需要统计表中的记录数使用COUNT(*)、COUNT(1)、COUNT(具体字段) 哪个效率更高呢
# 如果使用的是MyISAM 存储引擎则三者效率相同都是O(1)
# 如果使用的是InnoDB 存储引擎则三者效率COUNT(*) = COUNT(1)> COUNT(字段)
#其他方差、标准差、中位数

2. GROUP BY 的使用

需求查询各个部门的平均工资最高工资

SELECT department_id,AVG(salary),SUM(salary)
FROM employees
GROUP BY department_id

需求查询各个job_id的平均工资

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

需求查询各个department_id,job_id的平均工资

#方式1
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY  department_id,job_id;
#方式2
SELECT job_id,department_id,AVG(salary)
FROM employees
GROUP BY job_id,department_id;
#错误的
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id;
#结论1SELECT中出现的非组函数的字段必须声明在GROUP BY 中。
#      反之GROUP BY中声明的字段可以不出现在SELECT中。
#结论2GROUP BY 声明在FROM后面、WHERE后面ORDER BY 前面、LIMIT前面
#结论3MySQL中GROUP BY中使用WITH ROLLUP
SELECT department_id,AVG(salary)
FROM employees
GROUP BY department_id WITH ROLLUP;

需求查询各个部门的平均工资按照平均工资升序排列

SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;
#方式2
SELECT job_id,department_id,AVG(salary)
FROM employees
GROUP BY job_id,department_id;
#错误的
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id;
#结论1SELECT中出现的非组函数的字段必须声明在GROUP BY 中。
# 反之GROUP BY中声明的字段可以不出现在SELECT中。
#结论2GROUP BY 声明在FROM后面、WHERE后面ORDER BY 前面、LIMIT前面
#结论3MySQL中GROUP BY中使用WITH ROLLUP
SELECT department_id,AVG(salary)
FROM employees
GROUP BY department_id WITH ROLLUP;
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC;
#说明当使用ROLLUP时不能同时使用ORDER BY子句进行结果排序即ROLLUP和ORDER BY是互相排斥的。
#错误的
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id WITH ROLLUP
ORDER BY avg_sal ASC;

3. HAVING的使用 (作用用来过滤数据的)

练习查询各个部门中最高工资比10000高的部门信息

#错误的写法
SELECT department_id,MAX(salary)
FROM employees
WHERE MAX(salary) > 10000
GROUP BY department_id;
#要求1如果过滤条件中使用了聚合函数则必须使用HAVING来替换WHERE。否则报错。
#要求2HAVING 必须声明在 GROUP BY 的后面。
#正确的写法
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary) > 10000;
#要求3开发中我们使用HAVING的前提是SQL中使用了GROUP BY。

练习查询部门id为10,20,30,40这4个部门中最高工资比10000高的部门信息

方式1推荐执行效率高于方式2.
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2
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则此过滤条件必须声明在HAVING中。
#      当过滤条件中没有聚合函数时则此过滤条件声明在WHERE中或HAVING中都可以。但是建议大家声明在WHERE中。
/*
  WHERE 与 HAVING 的对比
1. 从适用范围上来讲HAVING的适用范围更广。 
2. 如果过滤条件中没有聚合函数这种情况下WHERE的执行效率要高于HAVING
*/

4. SQL底层执行原理

4.1 SELECT 语句的完整结构

/*
#sql92语法
SELECT ....,....,....(存在聚合函数)
FROM ...,....,....
WHERE 多表的连接条件 AND 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....
#sql99语法
SELECT ....,....,....(存在聚合函数)
FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件 
(LEFT / RIGHT)JOIN ... ON ....
WHERE 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....
*/

4.2 SQL语句的执行过程

#FROM ...,...-> ON -> (LEFT/RIGNT JOIN) -> WHERE -> GROUP BY -> HAVING -> SELECT -> DISTINCT ->

# ORDER BY -> LIMIT

课后练习

1.where子句可否使用组函数进行过滤? No!

2.查询公司员工工资的最大值最小值平均值总和

SELECT MAX(salary) max_sal ,MIN(salary) mim_sal,AVG(salary) avg_sal,SUM(salary) sum_sal
FROM employees;

3.查询各job_id的员工工资的最大值最小值平均值总和

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

4.选择具有各个job_id的员工人数

SELECT job_id,COUNT(*)
FROM employees
GROUP BY job_id;

5.查询员工最高工资和最低工资的差距DIFFERENCE #DATEDIFF

SELECT MAX(salary) - MIN(salary) "DIFFERENCE"
FROM employees;

6.查询各个管理者手下员工的最低工资其中最低工资不能低于6000没有管理者的员工不计算在内

SELECT manager_id,MIN(salary)
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary) >= 6000;

7.查询所有部门的名字location_id员工数量和平均工资并按平均工资降序

SELECT d.department_name,d.location_id,COUNT(employee_id),AVG(salary)
FROM departments d LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
GROUP BY department_name,location_id

8.查询每个工种、每个部门的部门名、工种名和最低工资

SELECT d.department_name,e.job_id,MIN(salary)
FROM departments d LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
GROUP BY department_name,job_id