【MySQL】子查询详解_ppppppatrick_mysql 子查询

网络投稿 02-07 2730

文章目录 0.概念1.需求分析与问题解决1.1提出具体问题：1.2 子查询的基本使用：子查询的基本语法结构： 1.3 子查询的分类 2.单行子查询2.1实例：2.2空值问题2.3非法使用子查询 3.多行子查询3.1多行比较操作符3.2代码实例3.3空值问题 4.相关子查询4.1代码实例4.2结论:4.3EXISTS 与 NOT EXISTS关键字 5.相关更新5.相关删除问题：谁的工资比Abel的高？

0.概念

子查询：一个查询语句嵌套在另一个查询语句内部

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

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

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 );

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。