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
posted @   carat9588  阅读(67)  评论(0编辑  收藏  举报
相关博文:
阅读排行:
· PowerShell开发游戏 · 打蜜蜂
· 在鹅厂做java开发是什么体验
· 百万级群聊的设计实践
· WPF到Web的无缝过渡:英雄联盟客户端的OpenSilver迁移实战
· 永远不要相信用户的输入:从 SQL 注入攻防看输入验证的重要性
点击右上角即可分享
微信分享提示