MySQL子查询_Hard Coder_mysql子查询

irpas 02-07 7762

一、子查询定义

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，这个特性从MySQL 4.1开始引入。

SQL 中子查询的使用大大增强了 SELECT 查询的能力，因为很多时候查询需要从结果集中获取数据，或者需要从同一个表中先计算得出一个数据结果，然后与这个数据结果（可能是某个标量，也可能是某个集合）进行比较。

二、子查询的基本使用

1、子查询（内查询）在主查询之前一次执行完成。 2、子查询的结果被主查询（外查询）使用。 3、注意事项：

（1）子查询要包含在括号内（2）将子查询放在比较条件的右侧（3）单行操作符对应单行子查询，多行操作符对应多行子查询

三、子查询分类

1、分类方式一

我们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。 2、分类方式二

我们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。

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。

四、单行子查询

1、单行比较操作符举例：

# 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 # 法一：(正常思路) 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;# 注意去除141号员工本身 # 法二： 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;# 注意去除141号员工本身

2、HAVING 中的子查询

#首先执行子查询，再向主查询中的HAVING 子句返回结果。 # 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 department_id,MIN(salary) FROM employees GROUP BY department_id # 只能想到使用group by...having,因为where中不能使用聚合函数 HAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id=50);

3、CASE中的子查询

# 在CASE表达式中使用单列子查询 # 显示员工的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" # CASE...WHEN...THEN...用在第一行 FROM employees;

4、子查询中的空值问题

SELECT last_name, job_id FROM employees WHERE job_id =( SELECT job_id FROM employees WHERE last_name = 'Haas');# 公司没有Haas这个人，子查询返回空值，最终结果也是空值，但不会报错

5、非法使用子查询

# 报错： Subquery returns more than 1 row # 原因：因为子查询的结果是多行数据，而父查询使用单行操作符=，不知道到底等于哪个数据 SELECT employee_id, last_name FROM employees WHERE salary =( SELECT MIN(salary) FROM employees GROUP BY department_id);

五、多行子查询

1、多行比较操作符举例：

# 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'; # 注意去除IT_PROG本身 # 返回其它job_id中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、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'; # 注意去除IT_PROG本身 # 查询平均工资最低的部门id # 法一 SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) = ( SELECT MIN(avg_sal) # 将子查询出来的数据作为一个临时表，一定要为所查询的列取别名和临时表取表名 FROM( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) t_dept_avg_sal ); # 法二 SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary)<=ALL( # 小于等于平均工资即相当于等于最小平均工资 SELECT AVG(salary) FROM employees GROUP BY department_id);

2、空值问题

SELECT last_name FROM employees WHERE employee_id NOT IN ( #返回结果为控制是因为子查询中查询出的manager_id有一个null值，导致not in判断有问题 SELECT manager_id FROM employees #WHERE manager_id IS NOT NULL );

六、相关子查询

1、相关子查询执行流程

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。举例：

# 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 # 法一：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); # 注意“本部门”关键字眼 # 法二：在from中声明子查询 # from型的子查询：子查询是作为from的一部分，子查询要用()引起来，并且要给这个子查询取别 #名，把它当成一张“临时的虚拟的表”来使用。 SELECT e.last_name,e.salary,e.department_id #注意此处要标明department_id所属的表，不然会报错(Column 'department_id' in field list is ambiguous) FROM employees e,( SELECT department_id,AVG(salary) avg_sal FROM employees GROUP BY department_id) 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 # department_name在departmnets表中 FROM departments d WHERE e.department_id=d.department_id); # 重要结论：在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询 # 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 e.employee_id,e.last_name,e.job_id FROM employees e WHERE 2<=(SELECT COUNT(*) FROM job_history WHERE employee_id=e.employee_id);

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。

举例：

# 查询公司管理者的employee_id，last_name，job_id，department_id信息 # 方式一：自连接 SELECT DISTINCT e1.employee_id,e1.last_name,e1.job_id,e1.department_id # 因为管理者可能管理多个员工，所以需要去重 FROM employees e1 JOIN employees e2 # 相当于求交集 WHERE e1.employee_id=e2.manager_id; # 方式二：子查询 SELECT employee_id,last_name,job_id,department_id FROM employees WHERE employee_id IN( SELECT DISTINCT manager_id FROM employees); # 方式三：exists SELECT e1.employee_id,e1.last_name,e1.job_id,e1.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：右连接 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; # employees表中没有的department_id # 方式二：not exists(与上面一题方式三类似) SELECT department_id,department_name FROM departments d WHERE NOT EXISTS ( SELECT * FROM employees e WHERE d.department_id= e.department_id );

七、小练习

#1.查询和Zlotkey相同部门的员工姓名和工资 SELECT last_name,salary FROM employees WHERE department_id IN ( SELECT department_id FROM employees WHERE last_name = 'Zlotkey' ); #2.查询工资比公司平均工资高的员工的员工号，姓名和工资。 SELECT employee_id,last_name,salary FROM employees WHERE salary > ( SELECT AVG(salary) FROM employees ); #3.选择工资大于所有JOB_ID = 'SA_MAN'的员工的工资的员工的last_name, job_id, salary SELECT last_name,job_id,salary FROM employees WHERE salary > ALL( SELECT salary FROM employees WHERE job_id = 'SA_MAN' ); #4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名 SELECT employee_id,last_name FROM employees WHERE department_id IN ( SELECT DISTINCT department_id FROM employees WHERE last_name LIKE '%u%' ); #5.查询在部门的location_id为1700的部门工作的员工的员工号 SELECT employee_id FROM employees WHERE department_id IN ( SELECT department_id FROM departments WHERE location_id = 1700 ); #6.查询管理者是King的员工姓名和工资 SELECT last_name,salary,manager_id FROM employees WHERE manager_id IN ( SELECT employee_id FROM employees WHERE last_name = 'King' ); #7.查询工资最低的员工信息: last_name, salary SELECT last_name,salary FROM employees WHERE salary = ( SELECT MIN(salary) FROM employees ); #8.查询平均工资最低的部门信息 #方式1： SELECT * FROM departments WHERE department_id = ( 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 * FROM departments WHERE department_id = ( 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： LIMIT SELECT * FROM departments WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary ) =( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ORDER BY avg_sal ASC LIMIT 1 ) ); #方式4： SELECT d.* FROM departments d,( SELECT department_id,AVG(salary) avg_sal FROM employees GROUP BY department_id ORDER BY avg_sal ASC LIMIT 0,1 ) t_dept_avg_sal WHERE d.`department_id` = t_dept_avg_sal.department_id #9.查询平均工资最低的部门信息和该部门的平均工资（相关子查询） #方式1： SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal FROM departments d WHERE department_id = ( 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 d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal FROM departments d WHERE department_id = ( 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： LIMIT SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal FROM departments d WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary ) =( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ORDER BY avg_sal ASC LIMIT 1 ) ); #方式4： SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal FROM departments d,( SELECT department_id,AVG(salary) avg_sal FROM employees GROUP BY department_id ORDER BY avg_sal ASC LIMIT 0,1 ) t_dept_avg_sal WHERE d.`department_id` = t_dept_avg_sal.department_id #10.查询平均工资最高的 job 信息 #方式1： SELECT * FROM jobs WHERE job_id = ( SELECT job_id FROM employees GROUP BY job_id HAVING AVG(salary) = ( SELECT MAX(avg_sal) FROM ( SELECT AVG(salary) avg_sal FROM employees GROUP BY job_id ) t_job_avg_sal ) ); #方式2： SELECT * FROM jobs WHERE job_id = ( SELECT job_id FROM employees GROUP BY job_id HAVING AVG(salary) >= ALL( SELECT AVG(salary) FROM employees GROUP BY job_id ) ); #方式3： SELECT * FROM jobs WHERE job_id = ( SELECT job_id FROM employees GROUP BY job_id HAVING AVG(salary) =( SELECT AVG(salary) avg_sal FROM employees GROUP BY job_id ORDER BY avg_sal DESC LIMIT 0,1 ) ); #方式4： SELECT j.* FROM jobs j,( SELECT job_id,AVG(salary) avg_sal FROM employees GROUP BY job_id ORDER BY avg_sal DESC LIMIT 0,1 ) t_job_avg_sal WHERE j.job_id = t_job_avg_sal.job_id #11.查询平均工资高于公司平均工资的部门有哪些? SELECT department_id FROM employees WHERE department_id IS NOT NULL GROUP BY department_id HAVING AVG(salary) > ( SELECT AVG(salary) FROM employees ); #12.查询出公司中所有 manager 的详细信息 #方式1：自连接 xxx worked for yyy 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` ); #13.各个部门中最高工资中最低的那个部门的最低工资是多少? #方式1： SELECT MIN(salary) FROM employees WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING MAX(salary) = ( SELECT MIN(max_sal) FROM ( SELECT MAX(salary) max_sal FROM employees GROUP BY department_id ) t_dept_max_sal ) ); SELECT * FROM employees WHERE department_id = 10; #方式2： SELECT MIN(salary) FROM employees WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING MAX(salary) <= ALL ( SELECT MAX(salary) FROM employees GROUP BY department_id ) ); #方式3： SELECT MIN(salary) FROM employees WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING MAX(salary) = ( SELECT MAX(salary) max_sal FROM employees GROUP BY department_id ORDER BY max_sal ASC LIMIT 0,1 ) ); #方式4： SELECT MIN(salary) FROM employees e,( SELECT department_id,MAX(salary) max_sal FROM employees GROUP BY department_id ORDER BY max_sal ASC LIMIT 0,1 ) t_dept_max_sal WHERE e.department_id = t_dept_max_sal.department_id #14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary #方式1： SELECT last_name, department_id, email, salary FROM employees WHERE employee_id = ANY ( SELECT DISTINCT manager_id FROM employees WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) = ( SELECT MAX(avg_sal) FROM ( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) t_dept_avg_sal ) ) ); #方式2： SELECT last_name, department_id, email, salary FROM employees WHERE employee_id = ANY ( SELECT DISTINCT manager_id FROM employees WHERE department_id = ( SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) >= ALL ( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) ) ); #方式3： SELECT last_name, department_id, email, salary FROM employees WHERE employee_id IN ( SELECT DISTINCT manager_id FROM employees e,( SELECT department_id,AVG(salary) avg_sal FROM employees GROUP BY department_id ORDER BY avg_sal DESC LIMIT 0,1 ) t_dept_avg_sal WHERE e.`department_id` = t_dept_avg_sal.department_id ); #15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号 #方式1： SELECT department_id FROM departments WHERE department_id NOT IN ( SELECT DISTINCT department_id FROM employees WHERE job_id = 'ST_CLERK' ); #方式2： SELECT department_id FROM departments d WHERE NOT EXISTS ( SELECT * FROM employees e WHERE d.`department_id` = e.`department_id` AND e.`job_id` = 'ST_CLERK' ); #16. 选择所有没有管理者的员工的last_name SELECT last_name FROM employees emp WHERE NOT EXISTS ( SELECT * FROM employees mgr WHERE emp.`manager_id` = mgr.`employee_id` ); #17．查询员工号、姓名、雇用时间、工资，其中员工的管理者为 'De Haan' #方式1： SELECT employee_id,last_name,hire_date,salary FROM employees WHERE manager_id IN ( SELECT employee_id FROM employees WHERE last_name = 'De Haan' ); #方式2： SELECT employee_id,last_name,hire_date,salary FROM employees e1 WHERE EXISTS ( SELECT * FROM employees e2 WHERE e1.`manager_id` = e2.`employee_id` AND e2.last_name = 'De Haan' ); #18.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资（相关子查询） #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 WHERE e.department_id = t_dept_avg_sal.department_id AND e.salary > t_dept_avg_sal.avg_sal #19.查询每个部门下的部门人数大于 5 的部门名称（相关子查询） SELECT department_name FROM departments d WHERE 5 < ( SELECT COUNT(*) FROM employees e WHERE d.department_id = e.`department_id` ); #20.查询每个国家下的部门个数大于 2 的国家编号（相关子查询） SELECT * FROM locations; SELECT country_id FROM locations l WHERE 2 < ( SELECT COUNT(*) FROM departments d WHERE l.`location_id` = d.`location_id` ); /* 子查询的编写技巧（或步骤）：① 从里往外写 ② 从外往里写如何选择？ ① 如果子查询相对较简单，建议从外往里写。一旦子查询结构较复杂，则建议从里往外写 ② 如果是相关子查询的话，通常都是从外往里写。 */

所用数据库请看文章末尾：SQL语言和基本的select语句