09 【子查询】

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09 【子查询】

子查询指一个查询语句嵌套在另一个查询语句内部的查询，这个特性从MySQL 4.1开始引入。

SQL 中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（可能是某个标量，也可能是某个集合）进行比较。

先写结论：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 ...,....

1. 需求分析与问题解决

1.1 实际问题

现有解决方式：

#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

子查询的基本语法结构：

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。

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。

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，那么这样的子查询叫做不相关子查询。

同样，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，这种嵌套的执行方式就称为相关子查询。

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：查询工资大于149号员工工资的员工的信息

题目：返回job_id与员工号为141的员工相同，salary比员工号为143的员工多的员工姓名，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

实现方式1：不成对比较

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);

实现方式2：成对比较

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);

2.3 HAVING 中的子查询

首先执行子查询。
向主查询中的HAVING 子句返回结果。

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

在CASE表达式中使用单列子查询：

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;

2.5 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');

子查询不返回任何行

2.6 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);

多行子查询使用单行比较符

3. 多行子查询

也称为集合比较子查询
内查询返回多行
使用多行比较操作符

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

体会 ANY 和 ALL 的区别

3.2 代码示例

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

题目：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

题目：查询平均工资最低的部门id

#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
				) 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
)


-- 方式3：
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC
LIMIT 1

3.3 空值问题

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
			SELECT manager_id  -- manager_id可能为空，not in(null,....)---->结果为空
			FROM employees
			);

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。

相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询。

说明：子查询中使用主查询中的列

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

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 
                   FROM employees 
                   GROUP BY department_id) e2
WHERE e1.`department_id` = e2.department_id
AND e2.dept_avg_sal < e1.`salary`;

from型的子查询：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。

在ORDER BY 中使用子查询：

题目：查询员工的id,salary,按照department_name 排序

-- 方式1
SELECT employee_id,salary
FROM employees e
ORDER BY (
	  SELECT department_name
	  FROM departments d
	  WHERE e.`department_id` = d.`department_id`
	);
	
	
-- 方式2：
SELECT employee_id,salary,department_name
FROM employees
JOIN departments
ON employees.department_id = departments.department_id
ORDER BY department_name;

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 e.employee_id, last_name,e.job_id
FROM   employees e 
WHERE  2 <= (SELECT COUNT(*)
             FROM   job_history 
             WHERE  employee_id = e.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：
- 条件返回 TRUE
- 不在子查询中继续查找
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

方式一：

SELECT employee_id, last_name, job_id, department_id
FROM   employees manager
WHERE  EXISTS ( SELECT *
                 FROM   employees worker
                 WHERE  worker.manager_id = 
                        manager.employee_id);

方式二：自连接

SELECT DISTINCT manager.employee_id,manager.last_name,manager.job_id,manager.department_id
FROM employees as work
JOIN employees manager
ON work.manager_id = manager.employee_id

方式三：

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

题目：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 'X'
                  FROM   employees
                  WHERE  department_id = d.department_id);

4.4 相关更新

UPDATE table1 alias1
SET    column = (SELECT expression
                 FROM   table2 alias2
                 WHERE  alias1.column = alias2.column);

使用相关子查询依据一个表中的数据更新另一个表的数据。

题目：在employees中增加一个department_name字段，数据为员工对应的部门名称

# 1）
ALTER TABLE employees
ADD(department_name VARCHAR2(14));
 
# 2）
UPDATE employees e
SET department_name =  (SELECT department_name 
	                       FROM   departments d
	                       WHERE  e.department_id = d.department_id);

4.5 相关删除

 DELETE FROM table1 alias1
 WHERE column operator (SELECT expression
                        FROM   table2 alias2
                        WHERE  alias1.column = alias2.column);

使用相关子查询依据一个表中的数据删除另一个表的数据。

题目：删除表employees中，其与emp_history表皆有的数据

DELETE FROM employees e
WHERE employee_id in  
           (SELECT employee_id
            FROM   emp_history 
            WHERE  employee_id = e.employee_id);

5. 抛一个思考题

**问题：**谁的工资比Abel的高？

解答：

#方式1：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2：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
		SELECT salary
		FROM employees
		WHERE last_name = 'Abel'
		);

**问题：**以上两种方式有好坏之分吗？

**解答：**自连接方式好！

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。

可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化。

练习

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.选择工资大于所有JOB_ID = 'SA_MAN'的员工的工资的员工的last_name, job_id, salary

-- 方式1 
SELECT last_name,job_id,salary
FROM employees
WHERE salary > (   -- 单行子查询
    SELECT MAX(salary) 
    FROM employees 
    WHERE job_id='SA_MAN' 
);
 
-- 方式2
SELECT last_name,job_id,salary 
FROM employees 
WHERE salary > ALL (  -- 多行子查询
    SELECT salary 
    FROM employees 
    WHERE job_id = 'SA_MAN' 
);

4.查询和姓名中包含字母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%'
	-- WHERE last_name REGEXP '[u]' --正则表达式
	);

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King的员工姓名和工资

-- 方式1 多表连接 join on
SELECT worker.last_name,worker.salary
FROM employees worker
JOIN employees mar
ON worker.manager_id = mar.employee_id
WHERE mar.last_name='King';
 
-- 方式2  多行子查询
SELECT last_name, salary 
FROM employees 
WHERE manager_id IN ( 
    SELECT employee_id 
    FROM employees 
    WHERE last_name = 'King' 
)

7.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
);

8.查询平均工资最低的部门信息

-- 方式一： 
SELECT * 
FROM departments 
WHERE department_id = ( 
		SELECT department_id 
		FROM employees 
		GROUP BY department_id 
		HAVING AVG(salary) = ( 
				SELECT MIN(dept_avgsal) 
				FROM (
						SELECT AVG(salary) dept_avgsal 
						FROM employees 
						GROUP BY department_id 
					) avg_sal 
		) 
);
 
-- 方式二： 
SELECT *
FROM departments
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) <= ALL (
		SELECT AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
	)
)
 
-- 方式三： 
SELECT *
FROM departments
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) = (
		SELECT AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal
		LIMIT 0, 1
	)
)
 
-- 方式四： 
SELECT d.*
FROM departments d, (
		SELECT department_id, AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal
		LIMIT 0, 1
	) dept_avg_sal
WHERE d.department_id = dept_avg_sal.department_id

9.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）

-- 方式一： 
SELECT d.*, (
		SELECT AVG(salary)
		FROM employees
		WHERE department_id = d.department_id
	) AS avg_sal
FROM departments d
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) = (
		SELECT MIN(dept_avgsal)
		FROM (
			SELECT AVG(salary) AS dept_avgsal
			FROM employees
			GROUP BY department_id
		) avg_sal
	)
);
 
-- 方式二： 
SELECT d.*
	, (
		SELECT AVG(salary)
		FROM employees
		WHERE department_id = d.`department_id`
	) AS avg_sal
FROM departments d
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) <= ALL (
		SELECT AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
	)
);
 
-- 方式三： 
SELECT d.*
	, (
		SELECT AVG(salary)
		FROM employees
		WHERE department_id = d.department_id
	) AS avg_sal
FROM departments d
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) = (
		SELECT AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal
		LIMIT 0, 1
	)
)
 
-- 方式四： 
SELECT d.*, dept_avg_sal.avg_sal
FROM departments d, (
		SELECT department_id, AVG(salary) AS avg_sal
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal
		LIMIT 0, 1
	) dept_avg_sal
WHERE d.department_id = dept_avg_sal.department_id

10.查询平均工资最高的 job 信息

-- 方式一： 
SELECT *
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary) = (
		SELECT MAX(avg_sal)
		FROM (
			SELECT AVG(salary) AS avg_sal
			FROM employees
			GROUP BY job_id
		) job_avgsal
	)
);
 
-- 方式二： 
SELECT *
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary) >= ALL (
		SELECT AVG(salary)
		FROM employees
		GROUP BY job_id
	)
);
 
-- 方式三： 
SELECT *
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees
	GROUP BY job_id
	HAVING AVG(salary) = (
		SELECT AVG(salary) AS avg_sal
		FROM employees
		GROUP BY job_id
		ORDER BY avg_sal DESC
		LIMIT 0, 1
	)
);
 
-- 方式四： 
SELECT j.*
FROM jobs j, (
		SELECT job_id, AVG(salary) AS avg_sal
		FROM employees
		GROUP BY job_id
		ORDER BY avg_sal DESC
		LIMIT 0, 1
	) job_avg_sal
WHERE j.job_id = job_avg_sal.job_id

11.查询平均工资高于公司平均工资的部门有哪些?

SELECT department_id
FROM employees
WHERE department_id IS NOT NULL
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

-- 方式1： 
SELECT employee_id, last_name, salary
FROM employees
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
);
 
-- 方式2： 
SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1
	JOIN employees e2
WHERE e1.employee_id = e2.manager_id;
 
-- 方式3： 
SELECT employee_id, last_name, salary
FROM employees e1
WHERE EXISTS (
	SELECT *
	FROM employees e2
	WHERE e2.manager_id = e1.employee_id
);

13.各个部门的最高工资中最低的那个部门的最低工资是多少?

-- 方式1: 
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) = (
		SELECT MIN(max_sal)
		FROM (
			SELECT MAX(salary) AS max_sal
			FROM employees
			GROUP BY department_id
		) dept_max_sal
	)
);
 
 
-- 方式2: 
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) <= ALL (
		SELECT MAX(salary) AS max_sal
		FROM employees
		GROUP BY department_id
	)
);
 
 
-- 方式3： 
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) = (
		SELECT MAX(salary) AS max_sal
		FROM employees
		GROUP BY department_id
		ORDER BY max_sal
		LIMIT 0, 1
	)
)
 
 
-- 方式4： 
SELECT employee_id, MIN(salary)
FROM employees e, (
		SELECT department_id, MAX(salary) AS max_sal
		FROM employees
		GROUP BY department_id
		ORDER BY max_sal
		LIMIT 0, 1
	) dept_max_sal
WHERE e.department_id = dept_max_sal.department_id

14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary

-- 方式一： 
SELECT employee_id, last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id = (
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary) = (
			SELECT MAX(avg_sal)
			FROM (
				SELECT AVG(salary) AS avg_sal
				FROM employees
				GROUP BY department_id
			) dept_sal
		)
	)
);
 
-- 方式二： 
SELECT employee_id, last_name, department_id, email, salary
FROM employees
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id = (
		SELECT department_id
		FROM employees e
		GROUP BY department_id
		HAVING AVG(salary) >= ALL (
			SELECT AVG(salary)
			FROM employees
			GROUP BY department_id
		)
	)
);
 
-- 方式三： 
SELECT *
FROM employees
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees e, (
			SELECT department_id, AVG(salary) AS avg_sal
			FROM employees
			GROUP BY department_id
			ORDER BY avg_sal DESC
			LIMIT 0, 1
		) dept_avg_sal
	WHERE e.department_id = dept_avg_sal.department_id
)

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

-- 方法一： 
SELECT department_id
FROM departments d
WHERE department_id NOT IN (
	SELECT DISTINCT department_id
	FROM employees
	WHERE job_id = 'ST_CLERK'
);
 
-- 方法二： 
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
	SELECT *
	FROM employees e
	WHERE d.`department_id` = e.`department_id`
		AND job_id = 'ST_CLERK'
);

16.选择所有没有管理者的员工的last_name

SELECT last_name
FROM employees e1
WHERE NOT EXISTS (
	SELECT *
	FROM employees e2
	WHERE e1.manager_id = e2.employee_id
);

17．查询员工号、姓名、雇用时间、工资，其中员工的管理者为 'De Haan'

-- 方式1： 
SELECT employee_id, last_name, hire_date, salary
FROM employees
WHERE manager_id = (
	SELECT employee_id
	FROM employees
	WHERE last_name = 'De Haan'
)
 
-- 方式2： 
SELECT employee_id, last_name, hire_date, salary
FROM employees e1
WHERE EXISTS (
	SELECT *
	FROM employees e2
	WHERE e2.`employee_id` = e1.manager_id
		AND e2.last_name = 'De Haan'
)

18.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（相关子查询）

-- 方式一：
SELECT employee_id, last_name, salary
FROM employees e1
WHERE salary > (
	SELECT AVG(salary)
	FROM employees e2
	WHERE e2.department_id = e1.`department_id`
);
 
-- 方式二： 
SELECT employee_id, last_name, salary
FROM employees e1, (
		SELECT department_id, AVG(salary) AS avg_sal
		FROM employees e2
		GROUP BY department_id
	) dept_avg_sal
WHERE e1.`department_id` = dept_avg_sal.department_id
	AND e1.`salary` > dept_avg_sal.avg_sal;

19.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）

-- 方式1
SELECT department_name
FROM departments d
WHERE 5 < (
	SELECT COUNT(*)
	FROM employees e
	WHERE d.department_id = e.department_id
);
  
-- 方式2
SELECT department_name
FROM departments
WHERE department_id IN (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING COUNT(*) > 5
);

20.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(*)
	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？

① 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

② 如果是相关子查询的话，通常都是从外往里写。

已到达文章底部，欢迎留言、表情互动~

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09 【子查询】

# 09 【子查询】

# 1. 需求分析与问题解决

# 1.1 实际问题

# 1.2 子查询的基本使用

# 1.3 子查询的分类

# 2. 单行子查询

# 2.1 单行比较操作符

# 2.2 代码示例

# 2.3 HAVING 中的子查询

# 2.4 CASE中的子查询

# 2.5 子查询中的空值问题

# 2.6 非法使用子查询

# 3. 多行子查询

# 3.1 多行比较操作符

# 3.2 代码示例

# 3.3 空值问题

# 4. 相关子查询

# 4.1 相关子查询执行流程

# 4.2 代码示例

# 4.3 EXISTS 与 NOT EXISTS关键字

# 4.4 相关更新

# 4.5 相关删除

# 5. 抛一个思考题

# 练习

预览:

09 【子查询】

1. 需求分析与问题解决

1.1 实际问题

1.2 子查询的基本使用

1.3 子查询的分类

2. 单行子查询

2.1 单行比较操作符

2.2 代码示例

2.3 HAVING 中的子查询

2.4 CASE中的子查询

2.5 子查询中的空值问题

2.6 非法使用子查询

3. 多行子查询

3.1 多行比较操作符

3.2 代码示例

3.3 空值问题

4. 相关子查询

4.1 相关子查询执行流程

4.2 代码示例

4.3 EXISTS 与 NOT EXISTS关键字

4.4 相关更新

4.5 相关删除

5. 抛一个思考题

练习