#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
#  单行操作符: > < >= <= <> !-
#  列子查询，一般搭配着多行操作符使用
#  IN,ANY/SOME(任意),ALL
# ④子查询的执行优先与主查询执行,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Abel高？
#①查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = 'Abel';

#②查询员工的信息，满足Salary>①结果
SELECT *
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,工资。
#①查141员工的job_id
SELECT job_id
FROM employees
WHERE employee_id='141';

#②查143员工的salary
SELECT salary
FROM employees
WHERE employee_id='143';

#③最后合并结果
SELECT CONCAT(last_name,first_name) AS 姓名,
job_id AS 工种编号,
salary AS 工资
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和salary。
SELECT MIN(salary)
FROM employees;

SELECT
last_name AS 姓,
salary AS 工资,
job_id AS 工种编号
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。
#①查50部门的最低工资
SELECT MIN(salary)
FROM employees
WHERE department_id=50;

#分组后，筛选条件①.【不用排除没有部门的所以不筛选部门编号】
SELECT department_id AS 部门编号,
MIN(salary) AS 月薪
FROM employees
#WHERE department_id
GROUP BY department_id
HAVING 月薪>(
 SELECT MIN(salary)
 FROM employees
    );

#2.列子查询(多行子查询)
#返回多行
#使用多行比较操作符

#案例1.返回location_id是1400或1700的部门中的所有员工姓名。
#①查询location_id是1400或1700的部门编号
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700);

#②查询员工姓名，要求部门号是①列表的某一个
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id IN (
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
 );

用ANY替代IN与上面同样的结果
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id = ANY(
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
 );

#案例.返回location_id不是1400或1700的部门中的所有员工姓名。
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id NOT IN(
 SELECT DISTINCT department_id
 FROM departments
 WHERE location_id IN(1400,1700)
);
==============================
SELECT CONCAT(last_name,first_name) AS 姓名
FROM employees
WHERE department_id <> 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部门任意一工资低的员工工号,
# 姓名,job_id以及salary
#①把IT_PROG部门中的工资查出来
SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';

#②把不是IT_PROG部门信息查出来
SELECT *
FROM employees
WHERE job_id != 'IT_PROG';

#③合并①与②在员工表中查出来
SELECT employee_id AS 员工编号,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工种编号,
salary AS 工资
FROM employees
WHERE job_id != 'IT_PROG'
AND salary<ANY(
 SELECT salary
 FROM employees
 WHERE job_id='IT_PROG'
  );

用MAX代替ANY与上面同样的效果
SELECT employee_id AS 员工编号,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工种编号,
salary AS 工资
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary<(
 SELECT MAX(salary)
 FROM employees
 WHERE job_id='IT_PROG'
  );

#案例3.返回其他部门中比job_id为‘IT_PROG'部门所有工资都低的员工
#的员工号，姓名，job_id以及salary。
#①先把IT_PROG部门的工资查出来。
SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';

SELECT employee_id AS 员工号,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工种编号,
salary AS 工资
FROM employees
WHERE salary<ALL(
 SELECT DISTINCT salary
 FROM employees
 WHERE job_id='IT_PROG'
)
 AND job_id <> 'IT_PROG';
=============================
MIN替代ALL
SELECT employee_id AS 员工号,
CONCAT(last_name,first_name) AS 姓名,
job_id AS 工种编号,
salary AS 工资
FROM employees
WHERE salary<(
　　SELECT MIN(salary)
　　FROM employees
　　WHERE job_id='IT_PROG'
)
 AND job_id <> 'IT_PROG';

#3.行子查询(结果集一行多列或者多行多列)
#案例1.查询员工编号最小并且工资最高的员工信息.引入
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
     );

这种查询结果使用虚拟字段，单行操作符必须一致可以使用。查出来与上面同样的效果。
SELECT *
FROM employees
WHERE (employee_id,salary)=(
 SELECT MIN(employee_id),
 MAX(salary)
 FROM employees
       );

#二.SELECT子查询
#仅仅支持标量子查询，结果是一行一列
#案例1.查询每个部门的员工个数
SELECT d.*,(SELECT COUNT(*) FROM employees)
FROM departments d; 

添加条件
SELECT d.*,(SELECT COUNT(*)
FROM employees e
WHERE e.department_id=d.department_id
) AS 个数
FROM departments d; 

#案例2.查询员工号=102的部门名。
SELECT department_name
FROM departments;
==============
SELECT employee_id
FROM employees
WHERE employee_id = 102;

SELECT employee_id,
(
 SELECT department_name
 FROM departments d
 WHERE e.department_id=d.department_id
)
FROM employees e
WHERE employee_id=102;

#三.FROM 后面
注意: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等级。
SELECT ROUND(AVG(salary),2),department_id
FROM employees
GROUP BY department_id;

SELECT e.平均工资,j.grade_level
FROM job_grades AS j
,(
SELECT ROUND(AVG(salary),2) AS 平均工资,department_id
 FROM employees
 GROUP BY department_id
) AS e
WHERE e.平均工资 BETWEEN j.lowest_sal AND j.highest_sal;

#1999语法，老师答案
SELECT e.*,j.grade_level
FROM (
　　SELECT ROUND(AVG(salary),2) AS 平均工资,department_id
　　FROM employees
　　GROUP BY department_id
) AS e
INNER JOIN job_grades j
ON e.平均工资 BETWEEN j.lowest_sal AND j.highest_sal;

#四.EXISTS后面(相关子查询)
语法:EXISTS(完整的查询语句)
备注:完整的查询语句可以是一行一列，可以使一行多列
注意:先走外查询，然后根据某个字段的值再去过滤
EXISTS 判断(布尔类型)值存不存在,结果只有两种:1有，0没有
#引入
SELECT EXISTS(SELECT employee_id FROM employees);

查询工资3W的员工信息
SELECT EXISTS(SELECT * FROM employees WHERE salary=30000);

#案例引入.查询员工名和部门名
#查员工名与部门编号
SELECT first_name,department_id
FROM employees
WHERE department_id;

#查部门名
SELECT department_name
FROM departments;

#查员工名与部门名
SELECT e.first_name,d.department_name
FROM employees e
INNER JOIN (  SELECT department_name,department_id
 FROM departments
) AS d
ON e.department_id=d.department_id;

#案例1..查有员工的部门名
SELECT department_name
FROM departments d
WHERE EXISTS(
 SELECT *
 FROM employees e
 WHERE d.department_id=e.department_id
    );

使用IN代替EXISTS，同样是上面的结果
SELECT department_name
FROM departments d
WHERE d.department_id IN(
 SELECT department_id
 FROM employees
     );

#案例2.查询没有女朋友的男神信息
#IN方法
SELECT *
FROM boys bo
WHERE bo.id NOT IN(
 SELECT boyfriend_id
 FROM beauty be
);
===============
#EXISTS方法
SELECT *
FROM boys bo
WHERE NOT EXISTS(
 SELECT boyfriend_id
 FROM beauty be
 WHERE bo.id=be.boyfriend_id
);

进阶9:联合查询
UNION 联合 合并:将多条查询语句的结果合并成一个结果。
语法:
 查询语句1
 UNION
 查询语句2
 UNION
 ...

应用场景:
要查询的结果来自于多个表，且多个表没有直接的连接关系，
但查询信息一致时。
网页搜索内容，内容从不同的表中检索联合起来返回给用户。

特点:
1.要求多条查询语句的查询列数是一致的。
2.要求多条查询语句的查询的每一列的类型和顺序最好一致。
3.使用UNION关键字默认去重，如果使用UNION ALL全部展示，包含重复项