mysql子查询如何运用
发布时间：2023-06-17 15:00:25 所属栏目：MySql教程 来源：
导读：这篇“mysql子查询如何应用”文章的知识点大部分人都不太理解，所以小编给大家总结了以下内容，内容详细，步骤清晰，具有一定的借鉴价值，希望大家阅读完这篇文章能有所收获，下面我们一起来看看这篇&ldqu
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这篇“mysql子查询如何应用”文章的知识点大部分人都不太理解，所以小编给大家总结了以下内容，内容详细，步骤清晰，具有一定的借鉴价值，希望大家阅读完这篇文章能有所收获，下面我们一起来看看这篇“mysql子查询如何应用”文章吧。 简介 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 分类 单行子查询：查询的结果只有1行 多行子查询：查询的结果有多行 相关(或关联)子查询：子查询中的参数需要用到主查询中的值 不相关(或非关联)子查询 案例1 # 1.查询工资比Abel高用户 # 方式1：需要2步 SELECT salary FROM employees WHERE last_name = 'Abel'; SELECT last_name, salary FROM employees WHERE salary > 11000; # 方式2：自连接 SELECT e2.last_name, e2.salary FROM employees e1, employees e2 WHERE e2.`salary` > e1.`salary` #多表的连接条件 AND e1.last_name = 'Abel'; # 方式3：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（或主查询） 内查询（或子查询） 单行比较操作符 mysql子查询如何应用 代码案例 # 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 # 方式1： 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; # 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 WHERE department_id IS NOT NULL GROUP BY department_id HAVING MIN(salary) > ( SELECT MIN(salary) FROM employees WHERE department_id = 110 ); # 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; # 子查询结果为null，则最后结果为null SELECT last_name, job_id FROM employees WHERE job_id = (SELECT job_id FROM employees WHERE last_name = 'Haas'); 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 多行比较操作符 mysql子查询如何应用 代码案例 # IN: SELECT employee_id, last_name FROM employees WHERE salary IN (SELECT MIN(salary) FROM employees GROUP BY department_id); # ANY 或 ALL: # 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 job_id <> 'IT_PROG' AND salary < ANY ( SELECT salary FROM employees WHERE 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 job_id <> 'IT_PROG' AND salary < ALL ( SELECT salary FROM employees WHERE job_id = 'IT_PROG' ); # 查询平均工资最低的部门id # MySQL中聚合函数是不能嵌套使用的。 # 方式1： SELECT department_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2： SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) <= ALL( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) # 子查询结果为null，则最后结果为null SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees ); 相关子查询 子查询中使用主查询中的列 mysql子查询如何应用 代码案例 # 查询员工中工资大于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 WHERE salary > ( SELECT AVG(salary) FROM employees ); # 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：使用相关子查询 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：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) t_dept_avg_sal # 取别名 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; 在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询 SELECT ....,....,....(存在聚合函数) FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件 (LEFT / RIGHT)JOIN ... ON .... WHERE 不包含聚合函数的过滤条件 GROUP BY ...,.... HAVING 包含聚合函数的过滤条件 ORDER BY ....,...(ASC / DESC ) LIMIT ...,.... cDNAF" class=" list-paddingleft-2"> EXISTS 与 NOT EXISTS关键字 1、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 2、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 3、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 4、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 代码案例 # 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` ) # EXISTS 与 NOT EXISTS关键字 # 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：使用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1： 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2： SELECT department_id, department_name FROM departments d WHERE NOT EXISTS ( SELECT * FROM employees e WHERE d.`department_id` = e.`department_id` ); （编辑：聊城站长网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
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