# 第09章_子查询

# 1. 由一个具体的需求,引入子查询
# 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
  AND e1.last_name = 'Abel';

SELECT e2.last_name, e2.salary
FROM employees e1
         JOIN employees e2
              ON e2.salary > e1.salary
WHERE 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（或主查询）、内查询（或子查询）

/*
    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 。
    注意事项
        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

 */

#不推荐
SELECT last_name, salary
FROM employees
WHERE (
          SELECT salary
          FROM employees
          WHERE last_name = 'Abel'
      ) < salary;

/*
# 3. 子查询的分类
 角度1：从内查询返回的结果的条目数
            单行子查询 vs 多行子查询

 角度2：内查询是否被执行多次
            相关子查询 vs 不相关子查询

 比如：相关子查询的需求：查询工资大于本部门平均工资的员工信息。
       不相关子查询的需求：查询工资大于本公司平均工资的员工信息。
 */

#子查询的编写技巧（或步骤）：① 从里往外写  ② 从外往里写

# 4. 单行子查询
# 4.1 单行操作符：=   !=   >   >=    <    <=
# 题目：查询工资大于149号员工工资的员工的信息

SELECT employee_id, last_name, salary
FROM employees
WHERE salary > (
    SELECT salary
    FROM employees
    WHERE employee_id = 149
    );

# 题目：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
    );

# 题目：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
    );

# 题目：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。
# 方式1：
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;

# 方式2：了解
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id, department_id) = (
    SELECT manager_id, department_id
    FROM employees
    WHERE employee_id = 141
    )
  AND employee_id <> 141;

# 题目：查询最低工资大于110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 = 110
    );

# 题目：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 location_id = 1800) THEN 'Canada'
           ELSE 'USA' END "location"
FROM employees;

# 4.2 子查询中的空值问题
SELECT last_name, job_id
FROM employees
WHERE job_id = (SELECT job_id
                FROM employees
                WHERE last_name = 'Haas'
    );

# 4.3 非法使用子查询
#错误：Subquery returns more than 1 row
SELECT employee_id, last_name
FROM employees
WHERE salary =(
    SELECT MIN(salary)
    FROM employees
    GROUP BY department_id
    );

# 5. 多行子查询
# 5.1 多行子查询的操作符： IN ANY ALL SOME(同ANY)

# 5.2 举例
# IN:
SELECT employee_id, last_name
FROM employees
WHERE salary IN (
    SELECT MIN(salary)
    FROM employees
    GROUP BY department_id
    );

# ANY / ALL:
# 题目：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'
    );

# 题目：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'
    );

# 题目：查询平均工资最低的部门id
#MySQL中聚合函数是不能嵌套使用的。
#方式1：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
    SELECT MIN(avg_sal)
    FROM(
        SELECT AVG(salary) 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) avg_sal
    FROM employees
    GROUP BY department_id
    );

# 5.3 空值问题
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
# 回顾：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
# 6.1
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (
    SELECT AVG(salary)
    FROM employees
    );


# 题目：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
# 方式1：使用相关子查询
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：

SELECT e.last_name,e.salary,e.department_id
FROM employees e ,(
    SELECT department_id,AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id
    ) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal;

# 题目：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
             ) ASC;

# 结论：在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询！
/*
  SELECT ....,....,....(存在聚合函数)
 FROM ....(LEFT / RIGHT)JOIN....ON 多表的连接条件
 (LEFT / RIGHT)JOIN .... ON ....
 WHERE 不包含聚合函数的过滤条件
 GROUP BY ....,....
 HAVING 包含聚合函数的过滤条件
 ORDER BY ....,....(ASC / DESC)
 LIMIT ....,....
 */

# 题目：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 *
FROM job_history;

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;

# 6.2 EXISTS 与 NOT EXISTS 关键字

# 题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
#方式1：自连接
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 e1
WHERE EXISTS(
    SELECT *
    FROM employees e2
    WHERE e1.employee_id = e2.manager_id
          );

# 题目：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,d.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 department_id,department_name
FROM departments d
WHERE NOT EXISTS(
    SELECT *
    FROM employees e
    WHERE d.department_id = e.department_id
          );

SELECT COUNT(*)
FROM departments;