标签:salary -- employees Day03 MySQL department id SELECT
使用工具:SQLyog
使用语言:SQL
学习日期:03
学习内容:
Day03
聚合函数
-- 聚合函数
-- 常用的聚合函数
/*
AVG / SUM
MAX / MIN
COUNT
*/
-- 1.1 AVG / SUM 只适用于数值类型的字段或者变量
SELECT AVG(salary),SUM(salary)
FROM employees;
-- 1.2 MAX / MIN 求最大最小值 可以适用于数值类型,字符串类型,日期时间类型的字段和变量
SELECT MAX(salary),MIN(salary)
FROM employees;
-- COUNT 计算指定字段在查询结果中出现的个数
-- 注意:COUNT不计算null值
SELECT COUNT(salary),COUNT(1),COUNT(*)
FROM employees;
-- 如果需要计算表中的记录数,使用COUNT(*),COUNT(1),COUNT(具体字段)哪一个效率更高
-- 如果使用的是MyISAM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)
-- GROUP BY的使用(分组操作)
-- 练习:查询各个部分的平均工资,最高工资
SELECT AVG(salary),SUM(salary),department_id
FROM employees
GROUP BY department_id;
-- 查询各个department_id job_id的平均工资
SELECT AVG(salary),department_id,job_id
FROM employees
GROUP BY department_id,job_id
-- 结论: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:GROUP BY声明在FROM后面,WHERE后面,ORDER BY前面,LIMINT前面
-- 结论:MySQL中GROUP BY中使用WITH ROLLUP
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id WITH ROLLUP;
-- 查询各个部门的平均工资,按照平均工资升序排列
SELECT department_id,AVG(salary) 'avg_salary'
FROM employees
GROUP BY department_id
ORDER BY avg_salary ASC;
-- HAVING的使用(用来过滤数据)
-- 1,如果过滤条件中使用了聚合函数,必须使用HAVING来替换WHERE,否则报错
-- 2,HAVING必须声明在GROUP BY的后面
-- 3,开发中我们使用HAVING的前提是SQL中使用了GROUP BY
-- 查询各个部门中最高工资比一万高的部门信息
SELECT MAX(salary),department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) > 10000
-- 查询部门ID未10,20,30,40中的最高工资比10000高的部门信息
-- 方式1:推荐使用,执行效率高于方式2
SELECT MAX(salary),department_id
FROM employees
WHERE department_id IN (10,20,30,40)
GROUP BY department_id
HAVING MAX(salary) > 10000;
-- 方式2
SELECT MAX(salary),department_id
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,则此过滤条件蓄婢声明在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中和HAVING中都可以,建议声明在WHERE中
/*
WHERE与HAVING的对比
1,适用范围:HAVING的范围更广
2,如果过滤条件没有聚合函数,WHERE的执行效率要高于HAVING
*/
-- SQL的底层原理
-- 1,SELECT语句的完整结构
/*
#SQL92写法
SELECT ......(存在聚合函数)
FROM .....
WHERE 多表的连接 AND 过滤条件,不包含聚合函数的
GROUP BY....
HAVING ...(过滤条件,可以包含聚合函数)
ORDER BY .....(ASC/DESC)
LIMIT ....
2
#SQL99写法
SELECT ......(存在聚合函数)
FROM .....(LEFT / RIGHT)JOIN .....ON多表的连接条件
(LEFT / RIGHT)JOIN .....ON .....
WHERE 多表的连接 AND 过滤条件,不包含聚合函数的
GROUP BY....
HAVING ...(过滤条件,可以包含聚合函数)
ORDER BY .....(ASC/DESC)
LIMIT ....
*/
-- 2,SQL语句的执行过程:
-- FROM ...--> ON --> (LEFT / RIGHT JOIN) --> WHERE --> GROUP BY --> HAVING --> SELECT --> DISTINCT --> ORDER BY --> LIMIT
聚合函数练习
#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),MIN(salary),AVG(salary),SUM(salary)
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 MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id
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) AS '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 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 department_name,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
子查询
-- 子查询
-- 需求:谁的工资比Abel的高
-- 方式1:
SELECT last_name,salary
FROM employees
WHERE last_name = "Abel"
SELECT last_name,salary
FROM employees
WHERE salary > 11000;
-- 方式2:自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.salary > e1.salary -- 多表的连接条件
AND e1.last_name = 'Abel';
-- 方式3:子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE last_name = "Abel"
);
-- 2:称谓的规范:外查询(主查询)和内查询(子查询)
/*
注意事项
子查询(内查询)在主查询之前一次执行完成。
子查询的结果被主查询(外查询)使用 。
子查询要包含在括号内
将子查询放在比较条件的右侧
单行操作符对应单行子查询,多行操作符对应多行子查询
*/
/*
3,子查询的分类
3.1:单行子查询 多行子查询(从内查询返回的结果条目数)
3.2:相关子查询 不相关子查询 (从内查询是否被执行多次)
例子:查询工资大于本部门平均工资的员工信息(相关子查询)
例子:查询工资大于本公司平均工资的员工信息(不相关子查询)
*/
-- 子查询编写技巧:①:从里往外写 ②:从外往里写
单行子查询
-- 4,单行子查询
-- 4.1:单行操作符:= > < >= <= <>
-- 练习:查询工资大于149号员工工资的员工信息
SELECT last_name,salary,employee_id
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE employee_id = 149
);
-- 练习:返回job_id与141员工相同,salary比143号员工多的员工姓名,job_id和工资
SELECT last_name,job_id,salary
FROM employees
WHERE job_id = (
SELECT job_id
FROM employees
WHERE employee_id = 141
)
AND salary > (
SELECT salary
FROM employees
WHERE employee_id = 143
);
-- 练习:返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
SELECT MIN(salary)
FROM employees
);
-- 练习:查询与141号员工的manager_id和department_id相同的其他员工的employee_id,manager_id,department_id
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
SELECT manager_id
FROM employees
WHERE employee_id = 141
)
AND department_id = (
SELECT department_id
FROM employees
WHERE employee_id = 141
)
AND employee_id <> 141;
-- 练习:查询最低工资大于50号部门最低工资的部门id和其最低工资
SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);
-- 练习:显式员工的employee_id,last_name和location。其中,
-- 若员工department_id与location_id为1800
-- 的department_id相同,则location为’Canada’,其余则为’USA’。
SELECT employee_id,last_name,(CASE department_id WHEN (SELECT department_id
FROM departments
WHERE department_id = 1800)
THEN 'Canada'
ELSE 'USA' END) AS "location"
FROM employees;
多行子查询
-- 5,多行子查询
/*
多行子查询操作符:IN / ANY / ALL / SOME(同ANY)
也称为集合比较子查询
内查询返回多行
使用多行比较操作符
*/
-- IN 举例:
SELECT employee_id,last_name
FROM employees
WHERE salary IN (
SELECT MIN(salary)
FROM employees
GROUP BY department_id
);
-- ANY 举例:
-- 返回其它job_id中比job_id为‘IT_PROG’部门任一工资低的员工的
-- 员工号、姓名、job_id 以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
)
-- ALL 举例:
-- 返回其它job_id中比job_id为‘IT_PROG’部门所有工资低的员工的
-- 员工号、姓名、job_id 以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ALL(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
)
-- 查询平均工资最低的部门id
-- FROM中也可以使用子查询
-- MySQL聚合函数不可以嵌套
-- 方式1
SELECT department_Id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(avg_sal)
FROM (
SELECT AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
) t_dept_avg_sal
)
-- 方式2
SELECT department_Id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
SELECT AVG(salary) AS avg_sal
FROM employees
GROUP BY department_id
)
-- 5.3 空值问题
-- 例子
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
);
-- 改正后
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
WHERE manager_id IS NOT NULL
);
-- 6,相关子查询
-- 查询员工中工资大于本公司平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
-- 查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE department_id = e1.`department_id`
);
-- 方式2:在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees e2
GROUP BY department_id
) e2
WHERE e.`department_id` = e2.department_id
AND e.`salary` > e2.avg_sal
-- 查询员工的id,salary,按照department_name 排序
SELECT employee_id,salary
FROM employees e
ORDER BY (
SELECT department_name
FROM departments d
WHERE e.`department_id` = d.`department_id`
) DESC;
-- 结论:在SELECT中,除了GROUP BY和LIMIT中不可以写子查询,其他位置都可以
/*
若employees表中employee_id与job_history表中employee_id相同的数目不小于2,输出这些相同
id的员工的employee_id,last_name和其job_id
*/
SELECT employee_id,last_name,job_id
FROM employees e
WHERE 2 <= (
SELECT COUNT(*)
FROM job_history j
WHERE e.`employee_id` = j.`employee_id`
)
-- EXISTS 与 NOT EXISTS关键字
-- 练习:查询公司管理者的employee_id,last_name,job_id,department_id信息
-- 方式1:
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;
-- 方式2:子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
);
-- 方式3:使用EXISTS
SELECT employee_id,last_name,job_id,department_id
FROM employees e
WHERE EXISTS (
SELECT *
FROM employees e1
WHERE e.`employee_id` = e1.`manager_id`
);
-- 查询departments表中,不存在于employees表中的部门的department_id和department_name
-- 方式1
SELECT d.department_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;
-- 方式2
SELECT d.department_id,department_name
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.`department_id` = e.`department_id`
);
标签:salary,--,employees,Day03,MySQL,department,id,SELECT 来源: https://blog.csdn.net/liqiiulei/article/details/122063511
本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享; 2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关; 3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关; 4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除; 5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。