7、子查询
一、子查询概述
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。
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
- 子查询要包含在括号内,建议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
SELECT 字段名 FROM 表名 WHERE 字段名1 = ( SELECT 字段名1 FROM 表名 WHERE 过滤条件);
SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'abcd' );
二、子查询的分类
- 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
。- 单行子查询查询的结果返回一条记录
- 多行子查询查询的结果返回多条记录
- 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。
- 子查询从数据表中查询了数据结果,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。
单行子查询
单行比较操作符
操作符 | 含义 |
---|---|
= | equal to 等于 |
> | greater than 大于 |
>= | greater than or equal to 大于等于 |
< | less than 小于 |
<= | less than or equal to 小于等于 |
<> | not equal to 不等于 |
-- 查询工资大于149号员工工资的员工的信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE employee_id = 149
);
-- 返回job_id于141号员工相同,salary比143号员工的多的员工姓名,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
-- 升序排序,取第一个
SELECT last_name,job_id,salary
FROM employees
ORDER BY salary ASC
LIMIT 1;
-- 子查询的方法
SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
SELECT MIN(salary) FROM employees
);
-- 查询与141号或174号员工的manager_id和department_id相同的其它员工的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 (
SELECT department_id
FROM employees
WHERE employee_id = 141
) AND employee_id <> 141;
-- 方式2
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
SELECT 字段名1, 分组函数(字段名)
FROM 表名
GROUP BY 字段名1
HAVING 分组函数1(字段名) = (
SELECT 分组函数1(字段名)
FROM 表名
WHERE 过滤条件);
- 首先执行子查询。
- 向主查询中的 HAVING 子句返回结果。
-- 查询最低工资大于110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 = 110
);
CASE中的子查询
SELECT 字段名
CASE WHEN (
SELECT 字段名1
FROM 表名
WHERE 过滤条件)
THEN 结果1
ELSE 结果2 END AS 别名
FROM 表名;
-- 显示员工的employee_id,last_name和location
-- 其中,若员工的department_id与location_id为1800的department_id相同,则location为'Canada',其余为'USA'
SELECT employee_id,last_name,
CASE WHEN department_id = (
SELECT department_id
FROM departments d JOIN locations l
ON d.location_id = l.location_id
WHERE l.location_id = 1800
) THEN 'Canada'
ELSE 'USA' END AS "location"
FROM employees;
多行子查询
多行子查询的基本语法结构
SELECT 字段名
FROM 表名
WHERE 字段名1 IN (
SELECT 字段名1
FROM 表名
WHERE 过滤条件);
- 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
SELECT employee_id,last_name
FROM employees
WHERE salary IN (
SELECT MIN(salary)
FROM employees
GROUP BY department_id
);
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 |
-- 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';
-- 方式2
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < (
SELECT MAX(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';
-- 方式2
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < (
SELECT MIN(salary)
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
-- 查新平均工资最低的部门id
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
SELECT MIN(avg_sal)
FROM (
SELECT AVG(salary) AS "avg_sal"
FROM employees
GROUP BY department_id
) AS t_dept_avg_sal
);
-- 方式2
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
SELECT AVG(salary)
FROM employees
GROUP BY department_id
);
-- 方式3
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC
LIMIT 1;
相关子查询
相关子查询的基本语法结构
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。
-- 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1:相关子查询
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2:FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) AS "avg_sal"
FROM employees
GROUP BY department_id
) AS t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal;
-- 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
FROM departments d
WHERE e.department_id = d.department_id
) ASC;
-- 若emploess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
);
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: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。
- 不在子查询中继续查找
- 条件返回 TRUE
-- 查询公司管理者的employee_id,last_name,job_id,department_id信息
-- 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: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,不存在与employees表中的部门的department_id和department_name
-- 方式1
SELECT d.department_id,department_name
FROM employees e
RIGHT JOIN departments d ON d.department_id = e.department_id
WHERE e.department_id IS NULL;
-- 方式2
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.department_id = e.department_id
);
子查询的编写技巧
- 如果子查询相对较简单,建议从外往内写。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
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