第09章子查询第09章子查询 1. 需求分析与问题解决 1.1 实际问题题目：谁的工资比Abel高？ 1.2 子查

第09章子查询

1. 需求分析与问题解决

1.1 实际问题

题目：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' 
);

1.2 子查询的基本使用

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

1.3 子查询的分类

分类方式1：

我们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2：

我们按内查询是否被执行多次，将子查询划分为相关(或关联)子查询和不相关(或非关联)子查询。

2. 单行子查询

2.1 单行比较操作符

操作符	含义
=	equal to
greater than
>=	greater than or equal to
<	less than
<=	less than or equal to
<>	not equal to

2.2 代码示例

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
);

2.3 HAVING 中的子查询

题目：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
GROUP BY department_id
HAVING MIN(salary) > (
    SELECT MIN(salary)
    FROM employees
    WHERE department_id = 50
);

2.4 CASE中的子查询

题目：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 'Canada' ELSE 'USA' END
) location
FROM employees;

2.5 子查询中的空值问题

mysql> SELECT last_name, job_id
    -> FROM employees
    -> WHERE job_id = (
    ->     SELECT job_id
    ->     FROM employees
    ->     WHERE last_name = 'Haas'
    -> );
Empty set (0.01 sec)

子查询不返回任何行

2.6 非法使用子查询

mysql> SELECT employee_id, last_name
    -> FROM employees
    -> WHERE salary = ( # 多行子查询使用单行比较符
    ->     SELECT MIN(salary)
    ->     FROM employees
    ->     GROUP BY department_id
    -> );
ERROR 1242 (21000): Subquery returns more than 1 row

3. 多行子查询

3.1 多行比较操作符

操作符	含义
IN	等于列表中的任意一个
ANY	需要和单行比较操作符一起使用，和子查询返回的某一个值比较
ALL	需要和单行比较操作符一起使用，和子查询返回的所有值比较
SOME	实际上是ANY的别名，作用相同，一般常使用ANY

3.2 代码示例

题目：查询平均工资最低的部门id

#方式1：
SELECT department_id /*平均工资中最低的部门的 id*/
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
    SELECT MIN(avg_sal) /*平均工资中最低的*/
    FROM (
        SELECT AVG(salary) avg_sal /*平均工资*/
        FROM employees 
        GROUP BY department_id 
    ) dept_avg_sal 
);

#方式2：
SELECT department_id 
FROM employees 
GROUP BY department_id
HAVING AVG(salary) <= ALL ( /*小于等于最低的*/
    SELECT AVG(salary) avg_sal /*平均工资*/
    FROM employees
    GROUP BY department_id 
);

MySQL中聚合函数是不能嵌套使用的。MIN( AVG(avg_sal) )

3.3 空值问题

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
    SELECT manager_id/* manager_id 有一个null值, 此时的查询结果为空*/
    FROM employees
    /*解决方法: WHERE manager_id IS NOT NULL*/
);
Empty set (0.01 sec)

4. 相关子查询

4.1 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关联子查询。

4.2 代码示例

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

方式一：相关子查询

SELECT last_name, salary, department_id
FROM employees e
WHERE salary > (
    SELECT AVG(salary)
    FROM employees
    WHERE department_id = e.department_id
);

方式二：在 FROM 中使用子查询

SELECT last_name,salary,e1.department_id
FROM employees e1,(
    SELECT department_id,AVG(salary) dept_avg_sal/*将此结果作为一个表;注意AVG(salary)必须用别名*/
    FROM employees
    GROUP BY department_id
) e2 
WHERE e1.`department_id` = e2.department_id 
AND e2.dept_avg_sal < e1.`salary`;

在ORDER BY 中使用子查询：

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 /*按照department_name排序*/
    FROM departments d
    WHERE e.`department_id` = d.`department_id` 
);

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

4.3 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。
如果在子查询中不存在满足条件的行：
- 条件返回 FALSE
- 继续在子查询中查找
如果在子查询中存在满足条件的行：
- 不在子查询中继续查找
- 条件返回 TRUE
NOT EXISTS关键字表示如果不存在某种条件，则返回TRUE，否则返回FALSE。

题目：查询公司管理者的employee_id，last_name，job_id，department_id信息

分析: 只要管理者自己的, 去除员工的

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 */*此处找到一条, 就不在子查询中继续查找了 [效率高] */
	       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：外连接

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：NOT EXISTS

SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
		SELECT */*这些部门有员工, 存在于employees表*/
		FROM employees e
		WHERE d.`department_id` = e.`department_id`
		);

题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。

5. 例题

5.1 查询平均工资最低的部门信息

#方式1：
SELECT */*平均工资中最小的部门的信息*/
FROM departments
WHERE department_id = (
    SELECT department_id/*平均工资中最小的部门的id*/
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary ) = (
        SELECT MIN(avg_sal)/*平均工资中最小的*/
        FROM (
            SELECT AVG(salary) avg_sal/*平均工资; 作为新表时,这里有两处别名要注意*/
            FROM employees
            GROUP BY department_id
            ) t_dept_avg_sal

        )
    );
    
#方式2：
SELECT *
FROM departments
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) <= ALL( /*小于所有的*/
						SELECT AVG(salary)
						FROM employees
						GROUP BY department_id
						)
			);    


#方式3： LIMIT
SELECT *
FROM departments
WHERE department_id = (
			SELECT department_id
			FROM employees
			GROUP BY department_id
			HAVING AVG(salary ) =(
						SELECT AVG(salary) avg_sal/*排序,取最小*/
						FROM employees
						GROUP BY department_id
						ORDER BY avg_sal ASC
						LIMIT 1		
						)
			);


#方式4：
SELECT d.*
FROM departments d,(
		SELECT department_id,AVG(salary) avg_sal /*直接把平均工资中最小的作为新表, 里面只有一条数据*/
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal ASC
		LIMIT 0,1
		) t_dept_avg_sal
WHERE d.`department_id` = t_dept_avg_sal.department_id

5.2 查询平均工资最低的部门信息和该部门的平均工资（相关子查询）

/*直接在 SELECT 处进行相关子查询 */
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d,(
		SELECT department_id,AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal ASC
		LIMIT 0,1
		) t_dept_avg_sal
WHERE d.`department_id` = t_dept_avg_sal.department_id

5.3 查询平均工资高于公司平均工资的部门有哪些?

/*如果过滤条件中使用了聚合函数，则必须使用HAVING来替换WHERE。*/
SELECT department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary) > (
			SELECT AVG(salary)
			FROM employees
			);

5.4 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号

#方式1：
SELECT department_id
FROM departments
WHERE department_id NOT IN (
			SELECT DISTINCT department_id/*job_id是"ST_CLERK"的部门*/
			FROM employees
			WHERE job_id = 'ST_CLERK'
			);

#方式2：相关子查询
SELECT department_id /*根据返回的字段,进行需求查找*/
FROM departments d
WHERE NOT EXISTS (
		SELECT * /*找到这条数据之后, 把所有字段返回给上层*/
		FROM employees e /*相当于把两张表给连接起来*/
		WHERE d.`department_id` = e.`department_id`
		AND e.`job_id` = 'ST_CLERK'
		);

5.5 查询每个部门下的部门人数大于 5 的部门名称（相关子查询）

SELECT department_name
FROM departments d
WHERE 5 < (
	   SELECT COUNT(*) /* "每个" 部门下的部门人数*/
	   FROM employees e
	   WHERE d.department_id = e.`department_id`
	  );

5.6 查询每个国家下的部门个数大于 2 的国家编号（相关子查询）

SELECT country_id
FROM locations l
WHERE 2 < ( 
            SELECT COUNT(*) /*国家下的部门个数*/
            FROM departments d
            WHERE l.location_id = d.location_id
            );

子查询的编写技巧（或步骤）：① 从里往外写 ② 从外往里写, 如何选择？

如果子查询相对较简单，建议从外往里写。一旦子查询结构较复杂，则建议从里往外写

如果是相关子查询的话，通常都是从外往里写。

第09章 子查询