【MySQL】子查询详解

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0.概念

子查询：一个查询语句嵌套在另一个查询语句内部

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1.需求分析与问题解决

1.1提出具体问题：

# 法一:效率低
SELECT last_name,salary
FROM employees
WHERE last_name = 'ABEL'
SELECT last_name,salary
FROM employees
WHERE salary > 11000;

#法二：自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.`salary` > e1.`salary`
AND e1.`last_name` = 'Abel';

#法三：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
		SELECT salary
		FROM employees
		WHERE last_name = 'ABEL'
               );


# 称谓的规范:外查询(主查询),内查询(子查询)

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

角度一:从内查询返回结果的条目数

单行子查询	多行子查询
子查询结果只有一个数据	子查询数据返回多个

角度二:内查询是否被执行多次

相关子查询	不相关子查询
查询工资大于本部门平均工资的员工信息	查询工资大于本公司平均工资的员工信息

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2.单行子查询

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

2.1实例：

# 查询工资大于149号员工工资的信息
SELECT salary,last_name,employee_id
FROM employees
WHERE salary > (
		SELECT salary 
		FROM employees
		WHERE employee_id = 149
		);
		
		
# 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号员工的manager_id和department_id相同的其他员工的employee_id，
# manager_id，department_id
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;

#方式二：成对查询
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;


# 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 MIN(salary),department_id,salary
FROM employees
GROUP BY department_id 
HAVING MIN(salary) > (
			SELECT MIN(salary)
			FROM employees
			WHERE department_id = 50
		     )
ORDER BY MIN(salary) DESC;

# 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;

2.2空值问题

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
);

内查询的结果是NULL空值,不会报错,但是也不会显示数据

2.3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);

这里内查询返回的结果是一列数据,不能使用等于号连接,必须使用in

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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

3.2代码实例

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

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

#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
相当于创建了一张临时的表

# 聚合函数不能嵌套，单行函数才能嵌套使用
SELECT employee_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);


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

3.3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
			 );

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4.相关子查询

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。

4.1代码实例

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

#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
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
		SELECT AVG(salary)
		FROM employees e2
		);

#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
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
		SELECT AVG(salary)
		FROM employees e2
		WHERE department_id = e1.`department_id`
		);

#方式二,在from中声明子查询
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) temp
WHERE e.department_id = temp.department_id
AND e.salary > temp.avg_sal;

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

SELECT employee_id,salary
FROM employees e1
ORDER BY (
	SELECT department_name
	FROM departments d 
	WHERE e1.`department_id` = d.`department_id`
	 );

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

#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`
	     );

4.2结论:

在哪里可以写子查询:

在select中,除了GROUP BY 和 LIMIT之外,其他位置都可以声明子查询

4.3EXISTS 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departments表中，不存在于employees表中的部门的department_id和department_name

#方式一：自连接
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：子查询
SELECT DISTINCT manager_id
FROM employees

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

#方式三：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`
	     )

查询departments表中，不存在于employees表中的部门的department_id和department_name

#方式一：
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;

#方式二：
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
		 SELECT *
		 FROM employees e
		 WHERE d.`department_id` = e.`department_id`
		 );

5.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);

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);

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。