进阶7：子查询
含义：
出现在其他语句中的select语句，称为子查询或内查询
外部的查询语句,称为主查询或外查询
eg：
select xxx
from xxx
where xxx in(select xxx
from xxx
where xxx)
分类：
按子查询出现的位置：
select后面：
仅仅支持标量子查询
from后面：
支持表子查询
where或having后面：★
标量子查询(单行)√
列子查询(多行)√
行子查询(用的较少)
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(结果集只有一列多行)
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,几行几列都可以)
#一、where或having后面
#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(多列多行)
/*
**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,一般搭配着单行操作符使用
< >= <= = <>**
列子查询，一般搭配着多行操作符使用
IN、ANY/SOME、ALL(返回true)
4子查询的执行优先于主查询执行,主查询的条件用到了子查询的结果
*/
#1、标量子查询
#案例1:谁的工资比Able高?
#1查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = ‘Able’;
#2查询员工的信息,满足salary>1结果
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SELECT *
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name = ‘Able’
);
#案例2：返回job_id与141号员工相同,salary比143号员工多的员工 姓名,job_id和工资
SELECT job_id
FROM employees
WHERE job_id=141
SELECT salary
FROM employees
WHERE salary=143
SELECT last_name,job_id,salary
FROM employees
WHERE job_id=(
SELECT job_id
FROM employees
WHERE job_id=141
)
AND salary>(
SELECT salary
FROM employees
WHERE salary=143
);
#案例3：返回公司工资最少的员工的last_name,job_id和salary
SELECT MIN(salary)
FROM employees
SELECT last_name,job_id,salary
FROM employees
WHERE salary=(
SELECT MIN(salary)
FROM employees
);
- #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
SELECT MIN(salary)
FROM employees
WHERE department_id=50
SELECT department_id,MIN(salary)
FROM employees
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id=50
);
#非法使用标量子查询
SELECT department_id,MIN(salary)
FROM employees
HAVING MIN(salary)>(
SELECT salary
FROM employees
WHERE department_id=250
);
#2、列子查询(多行子查询)——一列多行
#案例1：返回location_id是1400或1700的部门中的所有员工姓名
SELECT department_id
FROM departments
WHERE location_id IN(1400,1700)
SELECT last_name
FROM employees
WHERE department_id IN (
SELECT 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’部门任意工资低的员工的：工号、姓名、job_id、以及salary
#查询job_id为‘’部门任意工资
SELECT DISTINCT salary
FROM employees
WHERE job_id=‘it_prog’
#查询员工号、姓名、job_id 以及salary,salary<(1)的任意一个
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
SELECT DISTINCT salary
FROM employees
WHERE job_id=‘it_prog’
)AND job_id<>‘it_prog’;
#3、行子查询(结果集一行多列或多行多列)有局限性
SELECT *
FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
)
#案例：查询员工编号最小并且工资最高的员工信息
#查询最小的员工编号
SELECT MIN(employee_id)
FROM employees
SELECT MAX(salary)
FROM employees
SELECT *
FROM employees
WHERE employee_id=(
SELECT MIN(employee_id)
FROM employees
)
AND salary=(
SELECT MAX(salary)
FROM employees
);
#二、select后面(标量子查询 一行一列)
#案例：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 e.employee_id
FROM employees e
WHERE employee_id=102
SELECT (
SELECT department_name
FROM departments
ON d.department_id=e.department_id
WHERE e.employee_id=102
) 部门名;
;
#三、from后面(表子查询，多行多列)
将子查询结果充当一张表，要求必须起别名
#案例：查询每个部门的平均工资的工资等级
#1 查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees e
GROUP BY department_id
SELECT *
FROM job_grades;
#2 连接1的结果集和job_grades表，筛选条件在平均工资高between lowest_sal and highest_sal
SELECT ag_dep.*,g.grade_level
FROM(
SELECT AVG(salary) ag,department_id
FROM employees e
GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal
#四、exists后面(相关子查询)(表子查询，多行多列)(查询的结果有没有值1/0)
语法：
exists(完整的查询语句)
结果：
1或者0
SELECT EXISTS(SELECT employee_id FROM employees)
#案例1：查询有员工名的部门名
#in
SELECT department_name
FROM departments d
WHERE d.department_id IN(
SELECT department_id
FROM employees
)
SELECT d.department_name
FROM dpartments d
WHERE EXISTS(
SELECT *
FROM employees
WHERE d.department_id = e.department_id
);
#案例2：查询没有女朋友的男神信息
#in
SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
SELECT boyfriend_id
FROM beauty b
);
#exists(返回true、false)
SELECT bo.*
FROM boys bo
WHERE EXISTS(
SELECT b.*
FROM beauty b
WHERE b.boysfriend_id = bo.id
)
#text-----
#1查询和Zlotkey相同部门的员工姓名和工资
SELECT department_id
FROM departments
WHERE last_name = ‘Zlotkey’
SELECT last_name,salary
FROM employees
WHERE department_id=
(
SELECT department_name
FROM departments
WHERE last_name= ‘Zlotkey’
);
#2 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 AVG(salary)
FROM employees
SELECT employee_id,last_name,salary
FROM employees
WHERE salary>
(
SELECT AVG(salary)
FROM employees
)
#3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
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
#2连接1结果集 和employees表，进行筛选
SELECT employee_id,last_name,salary
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;
#4.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
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE ‘%u%’
SELECT last_name,employee_id
FROM employees
WHERE department_id IN
(
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE ‘%u%’
);
#5.查询在部门的location_id为1700的部门工作的员工的员工号
SELECT DISTINCT department_id
FROM department_id
WHERE location_id=1700
SELECT employee_id
FROM employees
LEFT JOIN departments d
WHERE department_id=ANY
(
SELECT DISTINCT department_id
FROM department_id
WHERE location_id=1700
);
#6.查询管理者是King的员工姓名和工资
SELECT manager_name
FROM employees
WHERE last_name=‘King’
SELECT last_name,salary*12
FROM employees
WHERE manager_name IN
(
SELECT manager_name
FROM employees
WHERE manager_name=‘King’
)
#7.查询工资最高的员工的姓名，要求first_name和
last_name显示为一列连接concat，列名为姓.名
SELECT MAX(salary)
FROM employees
SELECT CONCAT(first_name,last_name) “姓.名”
FROM employees
WHERE salary=
(
SELECT MAX(salary)
FROM e