9. 子查询

子查询是指一个查询语句嵌套在另一个查询语句内部的查询
子查询的使用大大增强了SELECT查询的能力，因为很多时候查询需要从结果集中获取数据，或者需要从同一个表中先计算得出一个数据结果，然后与这个数据结果进行比较

需求分析与问题解决

实际问题

问题：谁的工资比Abel高

//方式一
SELECT salary
FROM employees
WHERE last_name = 'Abel';
SELECT last_name,salary
FROM employees
WHERE salary > 11000;
//方式二：自连接
SELECT e2.last_name, e2.salary
FROM employees e1, employees e2
WHERE e1.last_name = 'Abel'
AND e1.salary < e2.salary
//方式三：子查询
SELECT last_name, salary
FROM employees
WHERE salary > (
  SELECT salary
  FROM employees
  WHERE last_name = 'Abel'
  );

子查询的基本使用

子查询：内查询
主查询：外查询
说明：

• 子查询在主查询之前一次执行完成
• 子查询的结果被主查询使用

• 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

    子查询的分类

• 分类方式一：

  按内查询的结果返回一条还是多条记录，将子查询分为单行子查询、多行子查询

• 分类方式二：

按内查询是否被执行多次，将子查询分为相关(或关联)子查询 和 不相关(或非关联)子查询

• 不相关子查询：子查询从数据表中查询了数据结果，如果这个数据结果只执行一次，然后这个数据结果作为主查询的条件进行执行
• 相关子查询：如果子查询需要执行多次，即采用循环的方式，先从外部查询开始，每次都传入子查询进行查询，然后再将结果反馈给外部

  单行子查询

  单行比较操作符：=、>、>=、<、<=、<>

  例题

  ```sql //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 );

//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 );

//查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_id， manager_id，department_id //方式一 SELECT employee_id, manager_id, department_id FROM employees WHERE manager_id IN ( SELECT manager_id FROM employees WHERE employee_id IN (174, 141) ) AND department_id IN ( SELECT department_id FROM employees WHERE employee_id IN (174, 141) ) AND employee_id NOT IN (174, 141);

//方式二 SELECT employee_id, manager_id, department_id FROM employees WHERE (manager_id, department_id) IN ( SELECT manager_id, department_id FROM employees WHERE employee_id IN (141, 174) ) AND employee_id NOT IN (141, 174);

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#### HAVING中的子查询
```sql
//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 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 = 50
);

CASE中的子查询

显示员工的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 'Canade' ELSE 'USA' END
) "location"
FROM employees;

非法使用子查询

SELECT employee_id, last_name
FROM employees
WHERE salary = (
  SELECT MIN(salary)
  FROM employees
  GROUP BY department_id
);

多行子查询

多行比较操作符

例题

//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 salary < ANY (
  SELECT salary
  FROM employees
  WHERE job_id = 'IT_PROG'
)
AND job_id <> 'IT_PROG';

//返回其它job_id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
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';

//查询平均工资最低的部门id
//方式一
SELECT department_id
FROM employees
GROUP BY department_id
HAVIGN AVG(salary) = (
  SELECT MIN(avg_sal)
  FROM (
    SELECT AVG(salary) avg_sal
    FROM employees
    GROUP BY department_id
  ) dept_avg_sal //Every derived table must have its own alias
);

//方式二
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (
  SELECT AVG(salary) avg_sal
  FROM employees
  GROUP BY department_id
);

空值问题

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
  SELECT manager_id
  FROM employees
);
answer: no rows selected
//原因：子查询的结果中有NULL，任何元素与NULL比较都是NULL


//正确写法
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
  SELECT manager_id
  FROM employees
  where manager_id is not NULL
);

相关子查询

执行流程

如果子查询的执行依赖于外部查询，通常情况下都是因为子查询中的表用到了外部的表，并进行了条件关联，因此每执行一次外部查询，子查询都要重新计算一次，这样的子查询就称之为相关子查询

例题

//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
//方法一，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
);

//方法二，在FROM中使用子查询
//说明：子查询是作为from的一部分，子查询要用()引起来
//并且要给这个子查询取别名， 把它当成一张“临时的虚拟的表”来使用
SELECT last_name, salary, e1.department_id
FROM employees e1, 
(SELECT department_id, AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2
where e1.department_id = e2.department_id
AND e1.salary > e2.dept_avg_sal

//查询员工的id, salary, 按照department_name排序
//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 employee_id,salary
FROM employees e
ORDER BY (
  SELECT department_name
  FROM departments d
  WHERE e.department_id = d.department_id
);

//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 employee_id, last_name, job_id
FROM employees e
WHERE 2 <= (
  SELECT COUNT(*)
  FROM job_history j
  WHERE e.employee_id = j.employee_id
);

EXISTS 与 NOT EXISTS关键字

关联子查询通常也会和 EXISTS操作符一起来使用，用来检查在子查询中是否存在满足条件的行

//查询公司管理者的employee_id，last_name, job_id, department_id信息
//方式一
SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id
FROM employees e1 JOIN employees e2
WHERE e1.employee_id = e2.manager_id;

//方式二
SELECT employee_id, last_name, job_id, department_id
FROM employees
WHERE employee_id IN (
  SELECT DISTINCT manager_id
  FROM employees
);

//方式三
SELECT employee_id, last_name, job_id, department_id
FROM employees e1
WHERE EXISTS ( 
  SELECT *
  FROM employees e2
  WHERE e2.manager_id = e1.employee_id
);

//查询departments表中，不存在于employees表中的部门的department_id和department_name
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (
  SELECT *
  FROM employees e
  WHERE d.department_id = e.department_id
);