#第08章_聚合函数
#1.几个常见的聚合函数
#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;
#如下的操作是没有意义的
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)
SELECT COUNT(employee_id),COUNT(salary),COUNT(2*salary),COUNT(1),COUNT(2),COUNT(*)
FROM employees;
#如果计算表中有多少条记录,如何实现?
#方式一:COUNT(*)
#方式二:COUNT(1)
#方式三: COUNT(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:查询公司中的平均奖金率
#错误的!
SELECT AVG(commission_pct)
FROM employees;
#正确的:
SELECT SUM(commission_pct)/COUNT(*),SUM(commission_pct)/COUNT(IFNULL(commission_pct,0)),
AVG(IFNULL(commission_pct,0))
FROM employees;
#如果需要统计表中记录数: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(具体字段)
#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,最高工资
SELECT job_id,AVG(salary),SUM(salary)
FROM employees
GROUP BY job_id;
#需求:查询各个department_id,job_id的平均工资,最高工资
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id
ORDER BY department_id ASC;
#或
SELECT job_id,department_id,AVG(salary)
FROM employees
GROUP BY job_id,department_id
ORDER BY department_id ASC;
#错误的!
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id;
#结论1:SELECT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2:GROUP BY声明在FROM之后、WHERE后面,ORDER BY前面,LIMIT前面
#结论3:MySQL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
#正确的
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC;
#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:用来过滤数据的
#查询各个部门中最高工资比10000高的部门信息
#错误的:
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2: HAVING必须声明在GROUP BY的后面
#正确的:
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary)>10000;
#开发中,我们使用HAVING的前提是SQL中使用了GROUP BY
#查询部门id为10,20,30,40这四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和HAVING中都可以,但建议声明在WHERE中。
/*
WHERE与HAVING的对比
1.从适用范围来说,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的执行效率高于HAVING
*/
#4.SQL底层执行原理
# 4.1SELECT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多表连接条件
JOIN...ON...
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 ...,...
*/
#4.2 sql语句的执行过程:
#FROM ...,...->ON->(LEFT/RIGHT JOIN)->WHERE->GROUP BY ->HAVING ->SELECT->DISTINCT->
#OEDER BY->LIMIT
#课后练习:
#1.where子句可否使用组函数进行过滤?
#不可以
#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) min_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) max_sal,MIN(salary) min_sal,AVG(salary) avg_sal,SUM(salary) sum_sal
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)
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
ORDER BY AVG(salary) DESC;
#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
mysql根据天聚合 mysql中聚合函数
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