#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，称为主查询或外查询
分类：
按子查询出现的位置：
select后面 仅标量子查询
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
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）
#----------------------------------------------------
#一，where和having后面
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：一般搭配多行操作符使用 in,any/some,all
4.子查询的执行优先于主查询
#1.标量子查询
#案例1：谁的工资比Abel高
SELECT last_name
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name='Abel'
);#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 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：返回公司工资最少的员工的 last_name,job_id,salary
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
#step 1.50号部门最低工资
SELECT MIN(salary)
FROM employees
WHERE department_id=50;
#step 2.最低工资大于1的部门id和其最低工资
SELECT department_id,MIN(salary) 最低工资
FROM employees
GROUP BY department_id
HAVING 最低工资>(
SELECT MIN(salary)
FROM employees
WHERE department_id=50
);#2.列子查询（多行子查询）
#案例1：返回location_id是1400或1700的部门中的所有员工姓名
#step 1 查询location_id是1400或1700的部门编号
SELECT department_id
FROM departments
WHERE location_id IN(1400,1700);
#step 2 查询员工姓名，要求部门号是1列表中的某一个
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’工种任一工资低的员工的员工号，姓名，job_id和工资
#step 1 job_id为IT_PROG的员工工资
SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';
#step 2 其他工种中比1任意工资低的员工信息
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ALL(
SELECT DISTINCT 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';#3.行子查询（结果集一行多列或多行多列） 较少使用
#案例：查询员工编号最小并且工资最高的员工信息
SELECT *
FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);#---------------------------------------------------------
#二，select后面
#仅仅支持标量子查询
#案例1：查询每个部门的员工个数
#分组查询不能得出员工个数为0的部门
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id;
#标量子查询
SELECT d.*,(
SELECT COUNT(*)
FROM employees e
WHERE e.department_id=d.`department_id`
)
FROM departments d;#案例2：查询员工号=102的部门名
#法一：多表连接
SELECT d.department_name,e.employee_id
FROM departments d
JOIN employees e
ON d.`department_id`=e.`department_id`
WHERE e.`employee_id`=102;
#法二：标量子查询
SELECT department_name
FROM departments
WHERE department_id=(
SELECT department_id
FROM employees
WHERE employee_id=102
);#-------------------------------------------------------------------
#三，from后面
#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
SELECT ag_dep.*,g.grade_level
FROM (
SELECT AVG(salary) 平均工资,department_id
FROM employees
GROUP BY department_id;
) ag_dep
JOIN job_grades g
ON ag_dep.平均工资 BETWEEN lowest_sal AND highest_sal#-------------------------------------------------------------------
#四，exists后面（相关子查询）
语法：
exists后面(完整的查询语句)
结果：
1或0
#案例1：查询有员工的部门名
#用列子查询
#step 1查询有员工的部门
SELECT DISTINCT department_id
FROM employees
WHERE department_id IS NOT NULL;
#step2 查询部门名
SELECT department_name
FROM departments d
WHERE d.department_id IN (
SELECT DISTINCT department_id
FROM employees
WHERE department_id IS NOT NULL
);#用exists
SELECT department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
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 DISTINCT be.boyfriend_id
FROM beauty be
);
#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT be.boyfriend_id
FROM beauty be
WHERE be.`boyfriend_id`=bo.`id`
);#---------------------------------------
#测试
#1.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'
);#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 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
#step 1查询各部门的平均工资
SELECT AVG(salary),department_id
FROM employees ep
GROUP BY department_id;
#step 2平均工资与员工表连接，并添加筛选条件
SELECT employee_id,last_name,salary
FROM employees e
JOIN (
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) avg_de
ON avg_de.department_id=e.`department_id`
WHERE e.salary>avg_de.ag;#4.查询姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT employee_id,last_name,department_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 employee_id
FROM employees
WHERE department_id IN (
SELECT department_id
FROM departments
WHERE location_id=1700
);#6.查询管理者是K_ing的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE manager_id IN (
SELECT employee_id
FROM employees
WHERE last_name='K_ing'
);#7.查询工资最高的员工姓名，要求first_name和last_name显示为一列，列名为 姓，名
SELECT CONCAT(first_name,',',last_name) '姓，名'
FROM employees
WHERE salary>=ALL(
SELECT salary
FROM employees
);#8.查询工资最低的员工信息：last_name,salary
SELECT last_name,salary
FROM employees
WHERE salary=(
SELECT MIN(salary)
FROM employees
);#9*.查询平均工资最低的部门信息
SELECT *
FROM departments
WHERE department_id=(
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary)
LIMIT 1
);#10.查询平均工资最高的job信息
SELECT *
FROM jobs
WHERE job_id=(
SELECT job_id
FROM employees
GROUP BY job_id
ORDER BY AVG(salary) DESC
LIMIT 1
);#11.查询平均工资高于公司平均工资的部门有哪些
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)>(
SELECT AVG(salary)
FROM employees
);#12.查询出公司中所有manager的详细信息
SELECT *
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
WHERE manager_id IS NOT NULL
);#13.查询各个部门最高工资中最低的那个部门的最低工资是多少
SELECT MIN(salary)
FROM employees
WHERE department_id=(
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY MAX(salary)
LIMIT 1
);