MySQL实例讲解子查询的使用

吾爱主题阅读：122 2023-03-02 16:08:00 评论：0

子查询-嵌套查询

子查询是指一个查询语句嵌套在另一个语句内部的查询

原始查询方法

1	`SELECT` `last_name,salaryFROM employeesWHERE last_name=` `'Abel'` `;` `SELECT` `last_name,salaryFROM employeesWHERE salary>11000;`

自连接

1	`SELECT` `e2.last_name,e2.salaryFROM employees e1,employees e2WHERE e2.salary>e1.salary` `AND` `e1.last_name=` `'Abel'` `;`

子查询

1	`SELECT` `last_name,salaryFROM employeesWHERE salary>(` `SELECT` `salaryFROM employeesWHERE last_name=` `'Abel'` `);`

称谓的规范：外查询（或主查询），内查询

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

子查询分类

单行子查询 vs 多行子查询

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和多行子查询

单行子查询

单行比较运算符

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

子查询的编写思路

从里往外写
从外往里写

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 SELECT * FROM employees WHERE salary>( SELECT salary FROM employees WHERE employee_id=149 ); 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 ); 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; 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;

HAVING中的子查询

先执行子查询
向主查询中的HAVING字句返回结果

1 2 3 4 5 6 7 8 SELECT department_id, MIN (salary) FROM employees GROUP BY department_id HAVING MIN (salary)>( SELECT MIN (salary) FROM employees WHERE department_id=50 );

CASE中的子查询

在CASE表达式中使用单列子查询

1 2	`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;`

子查询中的空值问题

若内查询查询结果为空的，则不会报错，子查询不会返回任何行

非法使用子查询

多行子查询

多行子查询也被称为集合比较子查询

内查询返回多行数据

使用多行比较操作符

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（任一）
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

MySQL中聚合函数是不能嵌套的

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 SELECT employee_id,last_name FROM employees WHERE salary IN ( SELECT MIN (salary) FROM employees GROUP BY department_id ); SELECT employee_id,last_name,job_id,salary FROM employees WHERE job_id<> 'IT_PROG' AND salary < ANY ( SELECT salary FROM employees WHERE job_id= 'IT_PROG' ); SELECT employee_id,last_name,job_id,salary FROM employees WHERE job_id<> 'IT_PROG' AND salary < ALL ( SELECT salary FROM employees WHERE job_id= 'IT_PROG' ); SELECT department_id FROM employees GROUP BY department_id HAVING AVG (salary) <= ALL ( SELECT AVG (salary) avgsal FROM employees GROUP BY department_id ); SELECT department_id FROM employees GROUP BY department_id HAVING AVG (salary)= ( SELECT MIN (avgsal) FROM ( SELECT AVG (salary) avgsal FROM employees GROUP BY department_id ) dbsal );