MySQL单表查询、多表查询、子查询

时间:2022-09-18 21:05:40

单表查询的语法

SELECT DISTINCT 字段1,字段2... FROM 表名
                  WHERE 条件
                  GROUP BY field
                  HAVING 筛选
                  ORDER BY field
                  LIMIT 限制条数

  

关键字的执行优先级

重点中的重点:关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit 

  

1.找到表:from

2.拿着where指定的约束条件,去文件/表中取出一条条记录

3.将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组

4.如果有聚合函数,则将组进行聚合

5.将4的结果过滤:having

6.查出结果:select

7.去重

8.将6的结果按条件排序:order by

9.将7的结果限制显示条数

 

MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
company.employee
    员工id      id                  int             
    姓名        emp_name            varchar
    性别        sex                 enum
    年龄        age                 int
    入职日期     hire_date           date
    岗位        post                varchar
    职位描述     post_comment        varchar
    薪水        salary              double
    办公室       office              int
    部门编号     depart_id           int



#创建表
create table employee(
id int not null unique auto_increment,
emp_name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);


#查看表结构
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field        | Type                  | Null | Key | Default | Extra          |
+--------------+-----------------------+------+-----+---------+----------------+
| id           | int(11)               | NO   | PRI | NULL    | auto_increment |
| emp_name         | varchar(20)           | NO   |     | NULL    |                |
| sex          | enum('male','female') | NO   |     | male    |                |
| age          | int(3) unsigned       | NO   |     | 28      |                |
| hire_date    | date                  | NO   |     | NULL    |                |
| post         | varchar(50)           | YES  |     | NULL    |                |
| post_comment | varchar(100)          | YES  |     | NULL    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+

#插入记录
#三个部门:教学,销售,运营
insert into employee(emp_name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

#ps:如果在windows系统中,插入中文字符,select的结果为空白,可以将所有字符编码统一设置成gbk

准备表和记录

建表和数据准备
建表和数据准备
#简单查询
    SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

    SELECT * FROM employee;

    SELECT emp_name,salary FROM employee;

#避免重复DISTINCT
    SELECT DISTINCT post FROM employee;    

#通过四则运算查询
    SELECT emp_name, salary*12 FROM employee;
    SELECT emp_name, salary*12 AS Annual_salary FROM employee;
    SELECT emp_name, salary*12 Annual_salary FROM employee;

#定义显示格式
   CONCAT() 函数用于连接字符串
   SELECT CONCAT('姓名: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary 
   FROM employee;
   
   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary 
   FROM employee;

   结合CASE语句:
   SELECT
       (
           CASE
           WHEN emp_name = 'jingliyang' THEN
               emp_name
           WHEN emp_name = 'alex' THEN
               CONCAT(emp_name,'_BIGSB')
           ELSE
               concat(emp_name, 'SB')
           END
       ) as new_name
   FROM
       emp;
MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
1 查出所有员工的名字,薪资,格式为
    <名字:egon>    <薪资:3000>
2 查出所有的岗位(去掉重复)
3 查出所有员工名字,以及他们的年薪,年薪的字段名为annual_year
 
select concat('<名字:', name,'>    <薪资:',salary, '>' )from employee;
 
select distinct post from employee;
 
select name, salary*12 AS annual_salary from employee;
小练习

 

where约束

where字句中可以使用:

1. 比较运算符:> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
    pattern可以是%或_,
    %表示任意多字符
    _表示一个字符 
5. 逻辑运算符:在多个条件直接可以使用逻辑运算符 and or not

#1:单条件查询
    SELECT emp_name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT emp_name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号,需要用IS)
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT emp_name,post_comment FROM employee 
        WHERE post_comment=''; 注意''是空字符串,不是null
    ps:
        执行
        update employee set post_comment='' where id=2;
        再用上条查看,就会有结果了

#5:关键字IN集合查询
    SELECT emp_name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT emp_name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT emp_name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'eg%';

    通配符’_’
    SELECT * FROM employee 
            WHERE emp_name LIKE 'al__';
MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
4. 查看岗位描述不为NULL的员工信息
5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
 
select name,age from employee where post='teacher';
 
select name,age from employee where post='teacher'and age>30;
 
SELECT
    name, age, salary
FROM
    employee
WHERE
    post = 'teacher'
        AND salary BETWEEN 9000 AND 10000;
 
 
select * from  employee where post_comment is not null;
 
select name,age,salary from employee where salary in(10000,9000,30000);
 
select name,age,salary from employee where salary not in(10000,9000,30000);
 
select name, salary*12 as annual_salary from employee where name like 'jin%';
小练习

 

group by 

单独使用GROUP BY关键字分组
    SELECT post FROM employee GROUP BY post;
    注意:我们按照post字段分组,那么select查询的字段只能是post,想要获取组内的其他相关信息,需要借助函数

GROUP BY关键字和GROUP_CONCAT()函数一起使用
    SELECT post,GROUP_CONCAT(emp_name) FROM employee GROUP BY post;#按照岗位分组,并查看组内成员名
    SELECT post,GROUP_CONCAT(emp_name) as emp_members FROM employee GROUP BY post;

GROUP BY与聚合函数一起使用
    select post,count(id) as count from employee group by post;#按照岗位分组,并查看每个组有多少人

强调:

如果我们用unique的字段作为分组的依据,则每一条记录自成一组,这种分组没有意义
多条记录之间的某个字段值相同,该字段通常用来作为分组的依据

  

聚合函数

#强调:聚合函数聚合的是组的内容,若是没有分组,则默认一组

示例:
    SELECT COUNT(*) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    SELECT MAX(salary) FROM employee;
    SELECT MIN(salary) FROM employee;
    SELECT AVG(salary) FROM employee;
    SELECT SUM(salary) FROM employee;
    SELECT SUM(salary) FROM employee WHERE depart_id=3;
MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
7. 查询男员工与男员工的平均薪资,女员工与女员工的平均薪资
 
 
#题1:分组
select post,group_concat(name) from employee group by post;
#题目2:
select post,count(id) from employee group by post;
#题目3:
select sex,count(id) from employee group by sex;
#题目4:
select post,avg(salary) from employee group by post;
#题目5
select post,max(salary) from employee group by post;
#题目6
select post,min(salary) from employee group by post;
#题目七
select sex,avg(salary) from employee group by sex;
小练习

 

  

HAVING过滤

HAVING与WHERE不一样的地方在于!!!!!!

#!!!执行优先级从高到低:where > group by > having 
#1. Where 发生在分组group by之前,因而Where中可以有任意字段,但是绝对不能使用聚合函数。
#2. Having发生在分组group by之后,因而Having中可以使用分组的字段,无法直接取到其他字段,可以使用聚合函数

  

MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
mysql> select @@sql_mode;
+--------------------+
| @@sql_mode         |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
row in set (0.00 sec)

mysql> select * from emp where salary > 100000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | emp_name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | alex | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
row in set (0.00 sec)

mysql> select * from emp having salary > 100000;
ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause

mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#错误,分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000;
+-----------+-------------------------------------------------------+
| post | group_concat(emp_name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
+-----------+-------------------------------------------------------+
rows in set (0.00 sec)

 验证
验证
MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
 
select name,count(id) from employee group by post having count(id)<2;
 
select post,avg(salary) from employee group by post having avg(salary)>10000;
  
select post,avg(salary) from employee group by post having avg(salary) between 10000 and 20000;
小练习

 

 

ORDER BY 查询排序

按单列排序
    SELECT * FROM employee ORDER BY salary;
    SELECT * FROM employee ORDER BY salary ASC;
    SELECT * FROM employee ORDER BY salary DESC;

按多列排序:先按照age排序,如果年纪相同,则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;

  

MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
1. 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
 
select * from employee group by age,hire_date desc;
 
select post, avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary);
  
select post, avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) desc;
小练习

 

LIMIT 限制查询的记录数

示例:
    SELECT * FROM employee ORDER BY salary DESC 
        LIMIT 3;                    #默认初始位置为0 
    
    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 0,5; #从第0开始,即先查询出第一条,然后包含这一条在内往后查5条

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 5,5; #从第5开始,即先查询出第6条,然后包含这一条在内往后查5条

  

使用正则表达式查询

SELECT * FROM employee WHERE emp_name REGEXP '^ale';

SELECT * FROM employee WHERE emp_name REGEXP 'on$';

SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';


小结:对字符串匹配的方式
WHERE emp_name = 'egon';
WHERE emp_name LIKE 'yua%';
WHERE emp_name REGEXP 'on$';

 

MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
查看所有员工中名字是jin开头,n或者g结果的员工信息
 
select * from employee where name regexp '^jin.*[n|g]$';
小练习

 

 

多表查询

MySQL单表查询、多表查询、子查询MySQL单表查询、多表查询、子查询
#建表
create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;


#查看表结构和数据
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

表department与employee

sql示例
建表

 

多表连接查询

#重点:外链接语法

SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

 

1 交叉连接:不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee,department;
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | egon       | male   |   18 |    200 |  200 | 技术         |
|  1 | egon       | male   |   18 |    200 |  201 | 人力资源     |
|  1 | egon       | male   |   18 |    200 |  202 | 销售         |
|  1 | egon       | male   |   18 |    200 |  203 | 运营         |
|  2 | alex       | female |   48 |    201 |  200 | 技术         |
|  2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|  2 | alex       | female |   48 |    201 |  202 | 销售         |
|  2 | alex       | female |   48 |    201 |  203 | 运营         |
|  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         |
|  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         |
|  4 | yuanhao    | female |   28 |    202 |  200 | 技术         |
|  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|  4 | yuanhao    | female |   28 |    202 |  203 | 运营         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |
|  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |
|  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |
|  6 | jingliyang | female |   18 |    204 |  200 | 技术         |
|  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |
|  6 | jingliyang | female |   18 |    204 |  202 | 销售         |
|  6 | jingliyang | female |   18 |    204 |  203 | 运营         |
+----+------------+--------+------+--------+------+--------------+

  

2 内连接:只连接匹配的行

#找两张表共有的部分,相当于利用条件从笛卡尔积结果中筛选出了正确的结果
#department没有204这个部门,因而employee表中关于204这条员工信息没有匹配出来
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;
+----+-----------+------+--------+--------------+
| id | name      | age  | sex    | name         |
+----+-----------+------+--------+--------------+
|  1 | egon      |   18 | male   | 技术         |
|  2 | alex      |   48 | female | 人力资源     |
|  3 | wupeiqi   |   38 | male   | 人力资源     |
|  4 | yuanhao   |   28 | female | 销售         |
|  5 | liwenzhou |   18 | male   | 技术         |
+----+-----------+------+--------+--------------+
 
#上述sql等同于
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

  

3 外链接之左连接:优先显示左表全部记录

#以左表为准,即找出所有员工信息,当然包括没有部门的员工
#本质就是:在内连接的基础上增加左边有右边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
+----+------------+--------------+
| id | name       | depart_name  |
+----+------------+--------------+
|  1 | egon       | 技术         |
|  5 | liwenzhou  | 技术         |
|  2 | alex       | 人力资源     |
|  3 | wupeiqi    | 人力资源     |
|  4 | yuanhao    | 销售         |
|  6 | jingliyang | NULL         |
+----+------------+--------------+

  

4 外链接之右连接:优先显示右表全部记录

#以右表为准,即找出所有部门信息,包括没有员工的部门
#本质就是:在内连接的基础上增加右边有左边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
+------+-----------+--------------+
| id   | name      | depart_name  |
+------+-----------+--------------+
|    1 | egon      | 技术         |
|    2 | alex      | 人力资源     |
|    3 | wupeiqi   | 人力资源     |
|    4 | yuanhao   | 销售         |
|    5 | liwenzhou | 技术         |
| NULL | NULL      | 运营         |
+------+-----------+--------------+

  

5 全外连接:显示左右两个表全部记录

全外连接:在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#注意:mysql不支持全外连接 full JOIN
#强调:mysql可以使用此种方式间接实现全外连接
select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id
;
#查看结果
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | egon       | male   |   18 |    200 |  200 | 技术         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|    2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |
+------+------------+--------+------+--------+------+--------------+
 
#注意 union与union all的区别:union会去掉相同的纪录

  

 符合条件连接查询

#示例1:以内连接的方式查询employee和department表,并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门
select employee.name,department.name from employee inner join department
    on employee.dep_id = department.id
    where age > 25;
 
#示例2:以内连接的方式查询employee和department表,并且以age字段的升序方式显示
select employee.id,employee.name,employee.age,department.name from employee,department
    where employee.dep_id = department.id
    and age > 25
    order by age asc;

  

子查询

#1:子查询是将一个查询语句嵌套在另一个查询语句中。
#2:内层查询语句的查询结果,可以为外层查询语句提供查询条件。
#3:子查询中可以包含:IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4:还可以包含比较运算符:= 、 !=、> 、<等

  

1 带IN关键字的子查询

#查询平均年龄在25岁以上的部门名
select id,name from department
    where id in
        (select dep_id from employee group by dep_id having avg(age) > 25);
 
#查看技术部员工姓名
select name from employee
    where dep_id in
        (select id from department where name='技术');
 
#查看不足1人的部门名(子查询得到的是有人的部门id)
select name from department where id not in (select distinct dep_id from employee);

 

2 带比较运算符的子查询

#比较运算符:=、!=、>、>=、<、<=、<>
#查询大于所有人平均年龄的员工名与年龄
mysql> select name,age from emp where age > (select avg(age) from emp);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
rows in set (0.00 sec)
 
 
#查询大于部门内平均年龄的员工名、年龄
select t1.name,t1.age from emp t1
inner join
(select dep_id,avg(age) avg_age from emp group by dep_id) t2
on t1.dep_id = t2.dep_id
where t1.age > t2.avg_age;

  

3 带EXISTS关键字的子查询

#department表中存在dept_id=203,Ture
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | egon       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | wupeiqi    | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+
 
#department表中存在dept_id=205,False
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=204);
Empty set (0.00 sec)

  

 SELECT语句关键字的定义顺序

SELECT DISTINCT <select_list>
FROM <left_table>
<join_type> JOIN <right_table>
ON <join_condition>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
ORDER BY <order_by_condition>
LIMIT <limit_number>