06-mysql基础-mysql中的DQL-子查询
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你好,我是在学mysql的小潭。在上一期的推文中我们学习了mysql的连接查询知识,本期我们将继续学习DQL中的子查询。
子查询介绍
含义:
出现在其他语句中的select语句,成为子查询或内查询
外部的查询语句,成为主查询或外查询
分类:
按子查询出现的位置:
select后面
仅支持标量子查询
from后面
支持表子查询
where或having后面
支持标量子查询(*)
列子查询(*)
行子查询
exists后面(相关子查询)
表子查询
按结果集的行列数不同:
标量子查询(结果集只有一行一列)
列子查询(结果集有一列多行)
行子查询(结果集有一行多列)
表子查询(结果集一般为多行多列)
其中where或having是常用到,其后可以是以下类型:
1.标量子查询(单行子查询)
2.列子查询(多行子查询)
3.行子查询(允许多行多列)
特点:
1.子查询放在小括号内
2.子查询一般放在条件的右侧
3.标量子查询,一般搭配单行操作符使用(>,<,>=,<=,=,<>条件运算符)
列子查询,一般搭配多行操作符使用(IN/NOT IN,ANY/SOME,ALL)
区别举例: a>any(10,20,30)等同于a>min(10,20,30)
a>all(10,20,30)等同于a>max(10,20,30)
4.子查询的执行优先于主查询的执行,主查询的条件用到了子查询的结果
具体示例:
标量子查询(where或having后)
#案例1:谁的工资比Abel高?
/*
思路:1.查询Abel的工资
2.查询员工的信息,满足salary>1的结果
*/
SELECT * FROM employees WHERE salary>
(
SELECT salary
FROM employees
WHERE last_name='Abel'
);
#案例2:返回job_id和141号员工相同,salary比143号员工多的员工姓名,job_id和工资
/*
思路:1.查询141号员工的job_id
2.查询143号员工的salary
3.查询姓名,job_id和工资,要求job_id=1的结果并且salary>2的结果
*/
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
);
#案例3:返回公司中工资最少的员工的last_name,job_id和salary
/*
思路:1.查询公司员工的最低工资
2.查询last_name,job_id,salary,要求salary=1的结果
*/
SELECT last_name,job_id,salary
FROM employees
WHERE salary =
(
SELECT MIN(salary) FROM employees
);
###############################################################
###############################################################
#标量子查询(having后)
#案例4: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
/*
思路:1.查询50号部门的最低工资
2.查询每个部门的最低工资
3.筛选2的结果中min(salary)>1的结果
*/
SELECT department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >
(
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);
非法使用标量子查询的情况:
#1.子查询结果量大于1
SELECT department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >
(
SELECT salary
FROM employees
WHERE department_id = 50
);
#2.子查询结果为空
SELECT department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >
(
SELECT MIN(salary)
FROM employees
WHERE department_id = 250
);
列子查询(多行子查询)
#案例1:返回location_id是1400或1700的部门中所有员工姓名
/*
思路:1.查询location是1400或1700的部门编号
2.查询员工姓名,要求部门号是1中的某一个
*/
SELECT last_name
FROM employees
WHERE department_id IN #此处in可以换做=ANY / 如果是not in可以换做<>ALL
(
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN (1400,1700)
);
#案例2:返回其他工种中比job_id为'IT_PROG'工种任一工资低的员工的工号,姓名,job_id和salary
/*
思路:1.查询job_id为'IT_PROG'部门任一工资
2.查询员工号,姓名,job_id和salary,要求salary<1的结果
*/
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary <
ANY(
SELECT salary FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
#或
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary <
(
SELECT MAX(salary) FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
#案例3:返回其他工种中比job_id为'IT_PROG'工种所有工资都低的员工的员工号,姓名,job_id和salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary <
ALL(
SELECT salary FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
#或
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary <
(
SELECT MIN(salary) FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
行子查询
#案例1:查询员工编号最小并且工资最高的员工信息
/*
传统思路:1.查询最小的员工编号
2.查询最高工资
3.查询员工信息
*/
SELECT *
FROM employees
WHERE employee_id =
(
SELECT MIN(employee_id)
FROM employees
) AND salary =
(
SELECT MAX(salary)
FROM employees
);
#使用行子查询代替
SELECT *
FROM employees
WHERE (employee_id,salary) =
(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);
select后的子查询
/*
仅支持标量子查询
*/
#案例1:查询每个部门的员工个数
SELECT d.*,
(SELECT COUNT(*) FROM employees e
WHERE e.department_id = d.`department_id`) 个数
FROM departments d;
#案例2:查询员工号=102的部门名
SELECT
(SELECT department_name FROM departments d
INNER JOIN employees e
ON d.department_id = e.department_id
WHERE e.employee_id = 102) 部门名;
from后的子查询
/*
将子查询结果充当一张表,要求必须起别名
*/
#案例1:查询每个部门的平均工资的工资等级
/*
思路:1.查询每个部门的平均工资
2.连接1的结果集和job_grades表,筛选条件是平均工资 between lowest_sal and highest_sal
*/
SELECT ag_dep.*,g.`grade_level`
FROM
(
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;
exists后的子查询(相关子查询)
/*
语法:exists(完整的查询语句)
结果:1或0
*/
#案例1:查询有员工的部门名
#in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN
(
SELECT department_id FROM employees
);
#exists用法
SELECT department_name
FROM departments d
WHERE EXISTS
(
SELECT *
FROM employees e
WHERE d.`department_id`=e.`department_id`
);
小测试:
#查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE department_id =
(
SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'
);
#查询工资比公司平均工资高的员工的员工号,姓名和工资
SELECT employee_id,last_name,salary
FROM employees
WHERE salary >
(
SELECT AVG(salary)
FROM employees
);
#查询各部门中工资比本部门平均工资高的员工的员工号,姓名和工资
/*
思路:1.查询各部门的平均工资
2.连接1结果集和employees表,再添加筛选
*/
SELECT employee_id,last_name,salary,e.`department_id`
FROM employees e
INNER JOIN
(
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) ag_dep
ON e.`department_id`=ag_dep.department_id
WHERE salary>ag_dep.ag;
#查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
/*
思路:1.查询姓名中包含字母u的员工的部门
2.查询部门号=1中的任意一个的员工号和姓名
*/
SELECT employee_id,last_name
FROM employees
WHERE department_id IN
(
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE '%u%'
);
#查询在部门的location_id为1700的部门工作的员工的员工号
/*
思路:1.查询locaion_id为1700的部门
2.查询部门号=1中任意一个的员工号
*/
SELECT employee_id
FROM employees
WHERE department_id IN #或使用=ANY
(
SELECT DISTINCT department_id
FROM departments
WHERE location_id = 1700
);
#查询管理者是K_ing的员工的姓名和工资
/*
思路:1.查询姓名为K_ing的员工编号
2.查询哪个员工的manager_id=1的结果
*/
SELECT last_name,salary
FROM employees
WHERE manager_id IN #当不确定子查询结果有多少个时,可以使用IN
(
SELECT employee_id
FROM employees
WHERE last_name='K_ing'
);
#查询工资最高的员工的姓名,要求first_name和last_name显示为一列,姓名为姓.名
/*
思路:1.查询最高工资
2.查询工资=1结果的姓名
*/
SELECT CONCAT(first_name,'.',last_name) "姓.名"
FROM employees
WHERE salary IN
(
SELECT MAX(salary)
FROM employees
);
下期预告:mysql中的查询4
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