1 需求分析与问题解决
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。
1.1 实际问题
现有解决方式：
#方式一： SELECT salary FROM employees WHERE last_name = 'Abel'; SELECT last_name,salary FROM employees WHERE salary > 11000; #方式二：自连接 SELECT e2.last_name,e2.salary FROM employees e1,employees e2 WHERE e1.last_name = 'Abel' AND e1.`salary` < e2.`salary` #方式三：子查询 SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );
1.2 子查询的基本使用
- 子查询的基本语法结构：
- 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 。
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
1.3 子查询的分类
1.3.1 分类方式1
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
1.3.2 分类方式2
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。
同样，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。
1.4 演示代码
#1. 由一个具体的需求，引入子查询 #需求：谁的工资比Abel的高？ #方式1（效率低，交互次数多）： SELECT salary FROM employees WHERE last_name = 'Abel'; /*输出： +----------+ | salary | +----------+ | 11000.00 | +----------+ */ SELECT last_name,salary FROM employees WHERE salary > 11000; /*部分输出： +-----------+----------+ | last_name | salary | +-----------+----------+ | King | 24000.00 | | Kochhar | 17000.00 | | De Haan | 17000.00 | | Greenberg | 12000.00 | | Russell | 14000.00 | | Partners | 13500.00 | */ #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（非等值连接） 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' ); #2. 称谓的规范：外查询（或主查询）、内查询（或子查询） /* - 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 */ #不推荐： SELECT last_name,salary FROM employees WHERE ( SELECT salary FROM employees WHERE last_name = 'Abel' ) < salary; /* 3. 子查询的分类 角度1：从内查询返回的结果的条目数 单行子查询 vs 多行子查询 角度2：内查询是否被执行多次 相关子查询 vs 不相关子查询 比如：相关子查询的需求：查询工资大于本部门平均工资的员工信息。（各部门平均工资不同且依赖于部门员工） 不相关子查询的需求：查询工资大于本公司平均工资的员工信息。（本公司平均工资是确定的一个数） 结合概念理解 */
2 单行子查询
2.1 单行比较操作符
2.2 代码示例
题目：查询工资大于149号员工工资的员工的信息
题目：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);
题目：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); /* +-----------+----------+---------+ | last_name | job_id | salary | +-----------+----------+---------+ | Olson | ST_CLERK | 2100.00 | +-----------+----------+---------+ */
题目：查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id
实现方式1：不成对比较
SELECT employee_id, manager_id, department_id FROM employees WHERE manager_id IN (SELECT manager_id FROM employees WHERE employee_id IN (174,141)) AND department_id IN (SELECT department_id FROM employees WHERE employee_id IN (174,141)) AND employee_id NOT IN(174,141);
实现方式2：成对比较
SELECT employee_id, manager_id, department_id FROM employees WHERE (manager_id, department_id) IN (SELECT manager_id, department_id FROM employees WHERE employee_id IN (141,174)) AND employee_id NOT IN (141,174);
2.3 HAVING中的子查询
- 首先执行子查询。
- 向主查询中的HAVING 子句返回结果。
题目：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 HAVING MIN(salary) > (SELECT MIN(salary) FROM employees WHERE department_id = 50);
2.4 CASE中的子查询
在CASE表达式中使用单列子查询：
题目：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;
2.5 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'); # Empty set (0.00 sec) #子查询不返回任何行
2.6 非法使用子查询
SELECT employee_id, last_name FROM employees WHERE salary = (SELECT MIN(salary) FROM employees GROUP BY department_id); #多行子查询使用单行比较符
演示代码
#子查询的编写技巧（或步骤）：① 从里往外写 ② 从外往里写 #4. 单行子查询 #4.1 单行操作符： = != > >= < <= #题目：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 ); /* +-------------+-----------+----------+ | employee_id | last_name | salary | +-------------+-----------+----------+ | 100 | King | 24000.00 | | 101 | Kochhar | 17000.00 | | 102 | De Haan | 17000.00 | | 108 | Greenberg | 12000.00 | | 114 | Raphaely | 11000.00 | | 145 | Russell | 14000.00 | | 146 | Partners | 13500.00 | | 147 | Errazuriz | 12000.00 | | 148 | Cambrault | 11000.00 | | 168 | Ozer | 11500.00 | | 174 | Abel | 11000.00 | | 201 | Hartstein | 13000.00 | | 205 | Higgins | 12000.00 | +-------------+-----------+----------+ */ #题目：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 ); /* +-------------+----------+---------+ | last_name | job_id | salary | +-------------+----------+---------+ | Nayer | ST_CLERK | 3200.00 | | Mikkilineni | ST_CLERK | 2700.00 | | Bissot | ST_CLERK | 3300.00 | | Atkinson | ST_CLERK | 2800.00 | | Mallin | ST_CLERK | 3300.00 | | Rogers | ST_CLERK | 2900.00 | | Ladwig | ST_CLERK | 3600.00 | | Stiles | ST_CLERK | 3200.00 | | Seo | ST_CLERK | 2700.00 | | Rajs | ST_CLERK | 3500.00 | | Davies | ST_CLERK | 3100.00 | +-------------+----------+---------+ */ #题目：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 ); /* +-----------+----------+---------+ | last_name | job_id | salary | +-----------+----------+---------+ | Olson | ST_CLERK | 2100.00 | +-----------+----------+---------+ */ #题目：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; /* +-------------+------------+---------------+ | employee_id | manager_id | department_id | +-------------+------------+---------------+ | 142 | 124 | 50 | | 143 | 124 | 50 | | 144 | 124 | 50 | | 196 | 124 | 50 | | 197 | 124 | 50 | | 198 | 124 | 50 | | 199 | 124 | 50 | +-------------+------------+---------------+ */ #方式2：了解 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; #题目：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 ); /* +---------------+-------------+ | department_id | MIN(salary) | +---------------+-------------+ | 70 | 10000.00 | | 90 | 17000.00 | +---------------+-------------+ */ #题目：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; /*部分输出 107行 +-------------+-------------+----------+ | employee_id | last_name | location | +-------------+-------------+----------+ | 100 | King | USA | | 101 | Kochhar | USA | | 102 | De Haan | USA | | 103 | Hunold | USA | | 104 | Ernst | USA | | 105 | Austin | USA | | 106 | Pataballa | USA | */ #4.2 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');#员工中没有Haas #4.3 非法使用子查询 #错误：Subquery returns more than 1 row SELECT employee_id, last_name FROM employees WHERE salary = (SELECT MIN(salary) FROM employees GROUP BY department_id);#返回值不止一个，不能用等号连接
3 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
3.1 多行比较操作符
3.2 代码示例
题目：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
题目：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
题目：查询平均工资最低的部门id
#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 ) 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 )
3.3 空值问题
SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees ); #输出：Empty set (0.00 sec) 原因：内查询有一个空值NULL #改正： SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees WHERE manager_id IS NOT NULL );
演示代码
#5.多行子查询 #5.1 多行子查询的操作符： IN ANY ALL SOME(同ANY) #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 #5.2举例： # IN: SELECT employee_id, last_name FROM employees WHERE salary IN (SELECT MIN(salary) FROM employees GROUP BY department_id); /*26条记录 +-------------+------------+ | employee_id | last_name | +-------------+------------+ | 101 | Kochhar | | 102 | De Haan | | 104 | Ernst | | 107 | Lorentz | | 113 | Popp | | 119 | Colmenares | */ # ANY / ALL: #题目：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' ); /*76条记录 +-------------+-------------+------------+---------+ | employee_id | last_name | job_id | salary | +-------------+-------------+------------+---------+ | 110 | Chen | FI_ACCOUNT | 8200.00 | | 111 | Sciarra | FI_ACCOUNT | 7700.00 | | 112 | Urman | FI_ACCOUNT | 7800.00 | | 113 | Popp | FI_ACCOUNT | 6900.00 | | 115 | Khoo | PU_CLERK | 3100.00 | | 116 | Baida | PU_CLERK | 2900.00 | */ #题目：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' ); /*4条记录 +-------------+-------------+----------+---------+ | employee_id | last_name | job_id | salary | +-------------+-------------+----------+---------+ | 115 | Khoo | PU_CLERK | 3100.00 | | 116 | Baida | PU_CLERK | 2900.00 | | 117 | Tobias | PU_CLERK | 2800.00 | | 118 | Himuro | PU_CLERK | 2600.00 | */ #题目：查询平均工资最低的部门id #注意：MySQL中聚合函数是不能嵌套使用的--如MIN(AVG(salary))是错误的 #注意：单行函数可以嵌套，Oracle中聚合函数可以嵌套 #方式1： SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) = ( SELECT MIN(avg_sal) FROM( #每一个子查询得到的表(AVG(salary))要有一个别名才可被外层查询使用 SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) t_dept_avg_sal ); /* +---------------+ | department_id | +---------------+ | 50 | +---------------+ */ #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 ); /*输出 +---------------+ | department_id | +---------------+ | 50 | +---------------+ */ #5.3 空值问题 SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees ); #输出：Empty set (0.00 sec) 原因：内查询有一个空值NULL #改正： SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees WHERE manager_id IS NOT NULL ); /* 输出： +-------------+ | last_name | +-------------+ | Ernst | | Austin | | Pataballa | */
4 相关子查询
4.1 相关子查询执行过程
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。
说明：子查询中使用主查询中的列
4.2 代码示例
题目：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
方式一：相关子查询
方式二：在 FROM 中使用子查询
SELECT last_name,salary,e1.department_id FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2 WHERE e1.`department_id` = e2.department_id AND e2.dept_avg_sal < e1.`salary`;
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。
在ORDER BY 中使用子查询：
题目：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` );
题目：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, last_name,e.job_id FROM employees e WHERE 2 <= (SELECT COUNT(*) FROM job_history WHERE employee_id = e.employee_id);
4.3 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：
- 不在子查询中继续查找
- 条件返回 TRUE
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。
题目：查询公司管理者的employee_id，last_name，job_id，department_id信息
方式一：
SELECT employee_id, last_name, job_id, department_id FROM employees e1 WHERE EXISTS ( SELECT * FROM employees e2 WHERE e2.manager_id = e1.employee_id);
方式二：自连接
SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id FROM employees e1 JOIN employees e2 WHERE e1.employee_id = e2.manager_id;
方式三：
SELECT employee_id,last_name,job_id,department_id FROM employees WHERE employee_id IN ( SELECT DISTINCT manager_id FROM employees );
题目：查询departments表中，不存在于employees表中的部门的department_id和department_name
SELECT department_id, department_name FROM departments d WHERE NOT EXISTS (SELECT 'X' FROM employees WHERE department_id = d.department_id);
4.4 相关更新
UPDATE table1 alias1 SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column);
使用相关子查询依据一个表中的数据更新另一个表的数据。
题目：在employees中增加一个department_name字段，数据为员工对应的部门名称
# 1） ALTER TABLE employees ADD(department_name VARCHAR2(14)); # 2） UPDATE employees e SET department_name = (SELECT department_name FROM departments d WHERE e.department_id = d.department_id);
4.5 相关删除
DELETE FROM table1 alias1 WHERE column operator (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column);
使用相关子查询依据一个表中的数据删除另一个表的数据。
题目：删除表employees中，其与emp_history表皆有的数据
DELETE FROM employees e WHERE employee_id in (SELECT employee_id FROM emp_history WHERE employee_id = e.employee_id);
演示代码
#6. 相关子查询 #回顾：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 #6.1 SELECT last_name,salary,department_id FROM employees WHERE salary > ( SELECT AVG(salary) FROM employees );#非相关子查询 #题目：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`#保证本部门 );#e1构成了相关性 /*输出 +-----------+----------+---------------+ | last_name | salary | department_id | +-----------+----------+---------------+ | King | 24000.00 | 90 | | Hunold | 9000.00 | 60 | | Ernst | 6000.00 | 60 | */ #方式2（非相关子查询）：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 /* +-----------+----------+---------------+ | last_name | salary | department_id | +-----------+----------+---------------+ | Hartstein | 13000.00 | 20 | | Raphaely | 11000.00 | 30 | | Weiss | 8000.00 | 50 | | Fripp | 8200.00 | 50 | | Kaufling | 7900.00 | 50 | */ #题目：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; /* +-------------+----------+ | employee_id | salary | +-------------+----------+ | 178 | 7000.00 | | 205 | 12000.00 | | 206 | 8300.00 | | 200 | 4400.00 | */ #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！ /* 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 ...,.... */ #题目：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 * FROM job_history; /* +-------------+------------+------------+------------+---------------+ | employee_id | start_date | end_date | job_id | department_id | +-------------+------------+------------+------------+---------------+ | 101 | 1989-09-21 | 1993-10-27 | AC_ACCOUNT | 110 | | 101 | 1993-10-28 | 1997-03-15 | AC_MGR | 110 | | 102 | 1993-01-13 | 1998-07-24 | IT_PROG | 60 | | 114 | 1998-03-24 | 1999-12-31 | ST_CLERK | 50 | | 122 | 1999-01-01 | 1999-12-31 | ST_CLERK | 50 | | 176 | 1998-03-24 | 1998-12-31 | SA_REP | 80 | | 176 | 1999-01-01 | 1999-12-31 | SA_MAN | 80 | | 200 | 1987-09-17 | 1993-06-17 | AD_ASST | 90 | | 200 | 1994-07-01 | 1998-12-31 | AC_ACCOUNT | 90 | | 201 | 1996-02-17 | 1999-12-19 | MK_REP | 20 | +-------------+------------+------------+------------+---------------+ */ 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` ) /*输出： +-------------+-----------+---------+ | employee_id | last_name | job_id | +-------------+-----------+---------+ | 101 | Kochhar | AD_VP | | 176 | Taylor | SA_REP | | 200 | Whalen | AD_ASST | +-------------+-----------+---------+ */ #6.2 EXISTS 与 NOT EXISTS关键字 #题目：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; /*18行 +-------------+-----------+---------+---------------+ | employee_id | last_name | job_id | department_id | +-------------+-----------+---------+---------------+ | 100 | King | AD_PRES | 90 | | 101 | Kochhar | AD_VP | 90 | | 102 | De Haan | AD_VP | 90 | | 103 | Hunold | IT_PROG | 60 | */ #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` ); #使用NOT EXISTS #题目：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; /* +---------------+----------------------+ | department_id | department_name | +---------------+----------------------+ | 120 | Treasury | | 130 | Corporate Tax | | 140 | Control And Credit | | 150 | Shareholder Services | */ #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` ); SELECT COUNT(*) FROM departments;
5 思考题
问题：谁的工资比Abel的高？
解答：
#方式1：自连接 SELECT e2.last_name,e2.salary FROM employees e1,employees e2 WHERE e1.last_name = 'Abel' AND e1.`salary` < e2.`salary` #方式2：子查询 SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'Abel'
问题：以上两种方式有好坏之分吗？
解答：自连接方式好！
题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。
可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化。
6 课后练习题
# 第09章_子查询的课后练习 #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：ALL 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。 */
