mysql-数据库增删改查
增删改查
mysql数据操作：DML
在mysql管理软件中，可以通过sql语句的DML语言来实现数据的操作：
- INSERT插入数据
- UPDATE更新数据
- DELETE删除数据
- SELECT查询数据
插入数据 insert
语法
1. 插入完整数据（顺序插入）
语法一：
INSERT INTO 表名(字段一，字段二，字段n) VALUES(值1，值2，值n);
语法二：
INSERT INTO 表名 VALUES(值1，值2，值n);
2. 指定字段插入数据
语法：
INSERT INTO 表名(字段2，字段3，字段n) VALUES(值1，值2，值3);
3. 插入多条记录
语法：
INSERT INTO 表民 VALUES
(值1，值2，值3),
(值1，值2，值3),
(值1，值2，值3);
4. 插入查询结果
语法：
INSERT INTO 表民(字段1，字段2，字段3) SELECT 字段1，字段2，字段3 FROM 表2;
更新数据 update
语法
语法：
UPDATE 表名 SET
字段1=值1，
字段2=值2，
WHERE 条件;
示例：
密码加密时password需要加密 所以需要使用password=password('明文密码')
正常加载数据时 只需要=新的值即可
UPDATE mysql.user SET password=password('123') WHERE user='root' and host='localhost';
删除数据 DELETE
语法
语法：
# 删除所有符合条件的行
DELETE FROM 表名 WHERE 条件;
示例：
DELETE FROM mysql.user WHERE password='';
delete from mysql.user where user='mhy' and host='localhost';
查询数据 SELECT
单表查询
单表查询语法
SELETE DISTINCT 字段1，字段2... FROM 表民
WHERE 条件
GROUP BY 字段
HAVING 筛选
ORDER BY 字段排序
LIMIT 限制条数
关键字执行的优先级
FROM # 1.找到表:from
WHERE # 2.拿着where指定的约束条件，去文件/表中取出一条条记录
GROUP BY # 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
SELECT # 4.执行select（去重）
DISTINCT # 4.执行select（去重）
HAVING # 5.将分组的结果进行having过滤
ORDER BY # 6.将结果按条件排序：order by
LIMIT # 7.限制结果的显示条数
数据准备
# 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 | |
+--------------+-----------------------+------+-----+---------+----------------+
10 rows in set (0.43 sec)
# 插入数据
#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
简单查询
# 查看数据
SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee;
SELECT * FROM employee;
select id,emp_name from employee;
# 避免重复查询
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，相当于python的字符串格式化
select concat('姓名: ',emp_name,' 年薪：',salary*12) as Anuual_salary from employee;
contat_ws() # 第一个参数为分隔符
select concat_ws(' : ',emp_name,salary*12) as Anuual_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
employee;
练习
- 查出所有员工的名字，薪资,格式为:
<名字:egon> <薪资:3000> - 查出所有的岗位（去掉重复）
- 查出所有员工名字，以及他们的年薪,年薪的字段名为annual_year
答案
select concat('<名字：',emp_name,'>','<薪资：',salary,'>') from employee;
select distinct post from employee;
select emp_name,salary*12 as annual_year from employee;
where约束
where语句中可以使用
- 比较运算符：> < >= <= = <> != (这两的意思都是不等于)
- between 80 and 100 值在80和100之间
- in (80,90,100) 值是80或者90或者100
- like 'e%'；通配符可以是%或者_
- % 表示任意多字符
- _ 表示任意一个字符
- 逻辑运算符：在多个条件中可以使用逻辑运算符 and or not
# 单条件查询
SELECT emp_name from employee where post = 'sale';
# 多条件查询
select emp_name from employee where post = 'sale' and salary > 3000;
# 关键字 between and
select emp_name from employee where salary BETWEEN 10000 AND 100000;
select emp_name from employee where salary not between 5000 and 2000000;
# 关键字is null (判断某个字段是否为null,需要用is)
select emp_name from employee where post_comment is null;
select emp_name 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
# 关键字 in 集合查询
select emp_name,salary from employee where salary=9000 or salary=8300;
select emp_name,salary from employee where salary in (9000,8300,4000.33);
select emp_name,salary from employee where salary not in (9000,8300,4000.33);
# 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__';
练习
- 查看岗位是teacher的员工姓名、年龄
- 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
- 查看岗位是teacher且薪资在9000-10000范围内的员工姓名、年龄、薪资
- 查看岗位描述不为NULL的员工信息
- 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
- 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
- 查看岗位是teacher且名字是jin开头的员工姓名、年薪
答案
select emp_name,age from employee where post = 'teacher';
select emp_name,age from employee where post='teacher' and age > 30;
select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select emp_name,salary*12 from employee where post='teacher' and emp_name like 'jin%';
group by 分组
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，需要用到函数
# group by 和 group_concat()函数一起使用：
# 按照岗位分组，并查看组内成员名
select post,group_concat(emp_name) from employee group by post;
select post,group_concat(emp_name) as name from employee group by post;
# group by 和聚合函数一起使用：
# 按照岗位分组，并查看每个组有多少人
select post,count(id) as count from employee group by post;
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数据，若是没有组，则默认一个组
# 计数函数 统计数据的条数
select count(*) from employee;
select count(*) from employee where depart_id = 1;
# 平均值函数 计算符合条件的值的平均值
select avg(salary) from employee;
# 相加函数 计算结果的和
select sum(salary) from employee;
select sum(salary) from employee where depart_id = 3;
# 最大值函数 返回值中最大的值
select max(salary) from employee;
# 最小值函数 返回值中最小的值
select min(salary) from employee;
练习
- 查询岗位名以及岗位包含的所有员工名字
- 查询岗位名以及各岗位内包含的员工个数
- 查询公司内男员工和女员工的个数
- 查询岗位名以及各岗位的平均薪资
- 查询岗位名以及各岗位的最高薪资
- 查询岗位名以及各岗位的最低薪资
- 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
答案
1. select post,group_concat(emp_name) as name from employee group by post;
2. select post,count(id) as name from employee group by post;
3. select sex,count(id) as sex_count from employee group by sex;
4. select post,avg(salary) as avg_salary from employee group by post;
5. select post,max(salary) as avg_salary from employee group by post;
6. select post,min(salary) as avg_salary from employee group by post;
7. select sex,avg(salary) as sex_salary from employee group by sex;
HAVING过滤
注: HAVING与WHERE不一样的地方在于!!!
! ! ! 优先级从高到底: where > group by > having
- where 发生在group by 分组之前, 因而where中可以有任何字段,但是绝对不能再使用聚合函数
- having 发生在group by 分组之后,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
测试
mysql> select @@sql_mode; # 查看全局的一些约束
+------------------------+
| @@sql_mode |
+------------------------+
| NO_ENGINE_SUBSTITUTION |
+------------------------+
1 row in set (0.00 sec)
mysql> select * from employee 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 |
| 19 | 麻花有 | female | 25 | 2019-09-22 | 中国外交部 | 中国未来全靠你们 | 999999.99 | 404 | 4 |
+----+-----------+--------+-----+------------+-----------------+--------------------------+------------+--------+-----------+
2 rows in set (0.00 sec)
# 错误，分组后无法直接取到salary字段
mysql> select post,group_concat(emp_name) from employee group by post HAVING salary > 100000;
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(emp_name) from employee group by post having avg(salary) > 100000;
+-----------------+--------------------------------------------------+
| post | group_concat(emp_name) |
+-----------------+--------------------------------------------------+
| teacher | jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
| 中国外交部 | 麻花有 |
+-----------------+--------------------------------------------------+
2 rows in set (0.00 sec)
练习
- 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
- 查询各岗位平均薪资大于10000的岗位名、平均工资
- 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
答案
1. select post,group_concat(emp_name) as name,count(id) as sum_id from employee group by post having sum_id < 2;
2. select post,avg(salary) as avg_salary from employee group by post having avg_salary > 10000;
3. 方法一
select post,avg(salary) as avg_salary from employee group by post HAVING avg_salary BETWEEN 10000 and 20000;
方法二
select post,avg(salary) as avg_salary from employee group by post having avg_salary > 10000 and avg_salary < 20000;
order by 查询排序
按单列排序：
select * from employee order by salary; # 默认升序
select * from employee order by salary asc; # 升序查询
select * from employee order by salary desc; # 倒序查询按多列排序：
先按照年龄排序（升序），再按照薪资排序起来（降序）
select * from employee order by age, salary desc;
练习
- 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
- 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
- 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
答案
1. select * from employee order by age asc,hire_date desc;
2. select post,avg(salary) as avg_salary from employee group by post having avg_salary > 10000 order by avg_salary asc;
3. select post,avg(salary) as avg_salary from employee group by post having avg_salary > 10000 order by avg_salary desc;
LIMIT 限制查询的记录数
示例：
# 默认初始位置为0，查询出从0开始到第3条；
select * from employee order by salary desc limit 3;
# 从第0条开始，既先查询出第一条，然后饱含这一条在内往后查询5条
select * from employee order by salary desc limit 1,5;
# 从第五条开始，既先查询第6条，然后饱含这一条在内往后查5条
select * from employee order by salary desc limit 5,5;
练习
- 分页显示，每页5条
答案
1. select * from employee limit 5;
2. select * from employee limit 5,5;
3. select * from employee limit 10,5;
使用正则表达式查询
select * from employee where emp_name regexp '^al';
select * from employee where emp_name regexp 'on$';
select * from employee where emp_name regexp '丫{1}';
小结：对字符串匹配的方式
WHERE emp_name = 'egon';
WHERE emp_name LIKE 'yua%';
WHERE emp_name REGEXP 'on$';
练习
查看所有员工中名字是jin开头，n或者g结果的员工信息
答案
1.select * from employee where emp_name regexp '^jin.*[ng]$';
多表查询
建表与数据准备
#建表
create table department(
id int,
name varchar(20)
);
create table empy(
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 empy(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 |
+----+------------+--------+------+--------+
查询语法
重点: 外链接语法
select 字段列表 from 表1 inner|left|right join 表二 on 表1.字段 = 表2.字段;
1 交叉链接: 不适用与任何匹配条件.生成笛卡尔积
select * from empy,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 |
+----+------------+--------+------+--------+------+--------------+
24 rows in set (0.00 sec)
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正确的结果
# department没有204这个部门,因而empy表中关于204这条员工的信息没有匹配出来
mysql> select empy.id,empy.name,empy.age,empy.sex,department.name from empy inner join department on empy.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 |
+----+-----------+------+--------+--------------+
5 rows in set (0.00 sec)
# 上述sql等价于
select empy.id,empy.name,empy.age,empy.sex,department.name from empy,department where empy.dep_id = department.id;
3 外链接之左外连接：优先显示左表全部记录
# 以左表为准,既找出所有员工信息,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 empy.id,empy.name,department.name from empy left join department on empy.dep_id = department.id;
+----+------------+--------------+
| id | name | 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 |
+----+------------+--------------+
6 rows in set (0.00 sec)
4 外连接之右外连接: 优先显示右表全部记录
# 以右表为准,既查找出所有部门信息,包括没有员工的部门
# 本质就是: 在内链接的基础上增加右边有左边没有的结果
mysql> select empy.id,empy.name,department.name from empy right join department on empy.dep_id = department.id;
+------+-----------+--------------+
| id | name | 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 |
+------+-----------+--------------+
6 rows in set (0.00 sec)
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可以使用此种方法间接实现全外连接
mysql> select * from empy left join department on empy.dep_id = department.id union select * from empy right join department on empy.dep_id = department.id;
+------+------------+--------+------+--------+------+--------------+
| 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 |
+------+------------+--------+------+--------+------+--------------+
7 rows in set (0.00 sec)
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
1. select empy.name,empy.age,empy.dep_id,department.id,department.name as de_name from empy inner join department on empy.age > 25 and empy.dep_id = department.id;
2. select empy.name,empy.age,empy.dep_id,department.id,department.name as de_name from empy inner join department on empy.dep_id = department.id where empy.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
1. select * from empy inner join department on empy.dep_id = department.id where 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,
department.NAME AS de_name
FROM
department
WHERE
id IN ( SELECT dep_id FROM empy GROUP BY dep_id HAVING avg( age ) > 25 )
#查看技术部员工姓名
SELECT NAME
FROM
empy
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 dep_id from empy);
select name from department where id not in (select distinct dep_id from empy);
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和年龄
mysql> select name,age from empy where age > (select avg(age) from empy);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
2 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
empy AS t1
INNER JOIN ( SELECT dep_id, avg( age ) AS avg_age FROM empy GROUP BY dep_id ) AS t2 ON t1.dep_id = t2.dep_id
WHERE
t1.age > t2.avg_age;
3 带EXISTS关键字的子查询
EXISTS 关键字表示存在. 在使用EXISTS关键字时,内层查询语句不返回查询的记录.
而是返回一个真假值. True或False
当返回True时,外层查询语句将进行查询;当返回为False时,外层语句不进行查询
# department表中存在dept_id=203，
# Ture 外层查询开始执行
# False 外层查询不执行
mysql> select * from empy where exists (select id from department where id = 203);
+----+------------+--------+------+--------+
| 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 |
+----+------------+--------+------+--------+
6 rows in set (0.00 sec)
mysql> select * from empy where exists (select id from department where id = 288);
Empty set (0.00 sec)
练习: 查询每个部门最新入职的那位员工
create table empy2(
id int not null unique auto_increment,
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
);
insert into empy2(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)
;
答案一
SELECT
*
FROM
empy2 AS t1
INNER JOIN ( SELECT post, max( hire_date ) AS max_date FROM empy2 GROUP BY post ) AS t2 ON t1.post = t2.post
WHERE
t1.hire_date = t2.max_date;
答案二
SELECT
t3.NAME,
t3.post,
t3.hire_date
FROM
empy2 AS t3
WHERE
id IN (
SELECT
( SELECT id FROM empy2 AS t2 WHERE t2.post = t1.post ORDER BY hire_date DESC LIMIT 1 )
FROM
empy2 AS t1
GROUP BY
post
);
答案一为正确答案，答案二中的limit 1有问题（每个部门可能有>1个为同一时间入职的新员工），我只是想用该例子来说明可以在select后使用子查询
可以基于上述方法解决：比如某网站在全国各个市都有站点，每个站点一条数据，想取每个省下最新的那一条市的网站质量信息
