掌握MySQL之查询语句的基本操作

mysql视频教程栏目介绍查询语句的操作 推荐： mysql视频教程 一.查询语句的基本操作 1.查询语句的基本操作 - select - from - where:约束条件 - group by:分组 - having：过滤 - distinct：去重 - orde

mysql视频教程栏目介绍查询语句的操作

推荐：mysql视频教程

一.查询语句的基本操作

1.查询语句的基本操作
- select
- from
- where:约束条件
- group by:分组
- having：过滤
- distinct：去重
- order by：排序
- limit：限制查询记录的数量
- 聚合函数: count（计数）
max（最大值）
min（最小值）
avg（平均值）
sum(求和)

二.单表查询

1、前期表与数据准备

# 创建一张部门表
create table emp(
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
);

# 插入记录
# 三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('tank','male',17,'20170301','张江第一帅形象代言部门',7300.33,401,1), # 以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('jason','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('大饼','male',18,'19000301','teacher',30000,401,1),
('sean','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

- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ G
- select * from empG- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ G
- select * from empG

重点：

写SQL语句必须遵循两点:
- 书写顺序:
# 获取id为 4、5的两条记录
select * from emp where id > 3 and id < 6;

- select
- from
- where

- 执行顺序:

比如: 图书管理员，得先找到是哪一个图书馆(哪张表),
再找这本书在图书馆的哪个位置(哪一条记录), 最后查找这个本书中某一页(哪些字段值);

select * from emp where id > 3 and id < 6;
- from ---> 找到图书馆
- where ---> 找到书的位置
- select ---> 找到书本中的某一页

注意: 必须记住SQL语句的 书写顺序 与 执行顺序(*******);

1.where(约束条件)

# PS: 根据执行顺序来书写 SQL语句，一步一步来写;

# 1.查询id大于等于3小于等于6的数据(and:与)

select * from emp where id >= 3 and id <= 6;

mysql> SELECT * FROM EMP WHERE ID > 3 AND ID <= 6;
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
3 rows in set (0.00 sec)

#可以使用between()and() :两者之间

mysql> select * from emp where id between 3 and 6;
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
4 rows in set (0.00 sec)

# 2.查询薪资是20000或者18000或者17000的数据
# or: 或者
select * from emp where salary=20000 or salary=18000 or salary=17000;
# in: 在什么里
select * from emp where salary in (20000, 18000, 17000);

mysql> select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
3 rows in set (0.00 sec)

# 也可以使用in(3,4,5,6)
select * from emp where id in(3,4,5,6);
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
4 rows in set (0.00 sec)

# 3.查询员工姓名中包含o字母 的 员工姓名和薪资
# like: 模糊匹配
# %: 匹配0个或多个任意字符(若前后都有%(%o%),查所有带有索引字符的记录,若头有%而尾没有(%o),则查所有尾带有索引字符的记录,若尾有%而头没有(o%),则查所有头带有索引字符的记录,若前后都没有%(o),则只会查询只有索引字符的字符)
# _: 匹配一个任意字符
select name, salary from emp where name like "%o%";
+-------+------------+
| name | salary |
+-------+------------+
| egon | 1000000.31 |
| jason | 3500.00 |
| owen | 2100.00 |
+-------+------------+
3 rows in set (0.00 sec)

mysql> select name, salary from emp where name like "o";
+------+--------+
| name | salary |
+------+--------+
| o | NULL |
+------+--------+
1 row in set (0.00 sec)

# 4.查找名字个数为3个的员工 名字 与 薪资
select name, salary from emp where name like "___";
+-----------+----------+
| name | salary |
+-----------+----------+
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
4 rows in set (0.00 sec)

# 或者 # char_length(name): 计算名字字符的长度
select name, salary from emp where char_length(name) = 4;

mysql> select name, salary from emp where char_length(name) = 3;
+-----------+----------+
| name | salary |
+-----------+----------+
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
4 rows in set (0.00 sec)

# 5.查询id小于3或者大于6的数据
# not in: 不再什么什么中
select * from emp where id not in (3, 4, 5, 6);
select * from emp where id not between 3 and 6;

# 6.查询薪资不在20000，18000，17000范围的数据
select * from emp where salary not in (20000, 18000, 17000);

# 7.查询岗位描述为空的 员工名 与 岗位名 post_comment
# 用等于号无法判定空字段
select name, post from emp where post_comment = null;

select * from emp where post_comment = null;
Empty set (0.00 sec)

# 注意: 针对null的值 需要使用 is
select name, post from emp where post_comment is null;

select name, post_comment from emp where post_comment is null;
+-----------+--------------+
| name | post_comment |
+-----------+--------------+
| tank | NULL |
| egon | NULL |
| kevin | NULL |
| jason | NULL |
| owen | NULL |
| jerry | NULL |
| 大饼 | NULL |
| sean | NULL |
| 歪歪 | NULL |
| 丫丫 | NULL |
| 丁丁 | NULL |
| 星星 | NULL |
| 格格 | NULL |
| 张野 | NULL |
| 程咬金 | NULL |
| 程咬银 | NULL |
| 程咬铜 | NULL |
| 程咬铁 | NULL |
| o | NULL |
+-----------+--------------+
19 rows in set (0.00 sec)

2.group by(分组)

- 书写顺序:
- select
- from
- where
- group by

- 执行顺序:
- from
- where
- group by
- select
# 什么叫分组
# 比如: 一张员工表中有性别字段，可以根据性别分组，一组是男性，一组是女性，或者是根据部门分组，有教学部、销售部等...

# 1.根据部门分组
# 非严格模式下可以获取 分组条件post 以外的字段数据
select post, salary from emp group by post;

# 设置严格模式:
show variables like "%mode%";
# 全局设置: 永久有效
set global sql_mode="strict_trans_tables,only_full_group_by";

"""
设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据，
不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，并且会报错,因为不分组就是对单个元素信息的随意获取
"""
mysql> select post, salary from emp group by post;
ERROR 1046 (3D000): No database selected

# 可以同聚合函数，间接获取其他字段数据
聚合函数:
count: 计数
max: 最大值
min: 最小值
avg: 平均值
sum: 求和
group_concat(name): 可以将分组后的 所有名字获取并进行拼接

# 指定以:拼接
select post, group_concat(name) from emp group by post;
select post, group_concat(name, ':') from emp group by post;

2.获取每个 部门 的最高工资
select post,max(salary) from emp group by post;
mysql> select post,max(salary) from emp group by post;
+-----------------------------------+-------------+
| post | max(salary) |
+-----------------------------------+-------------+
| NULL | NULL |
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 张江第一帅形象代言部门 | 7300.33 |
+-----------------------------------+-------------+
5 rows in set (0.00 sec)

补充: as 别名: 可以给字段 加一个 别名
select post as '部门', max(salary) as '薪资'
from emp group by post;

# 也可以简写,但是不推荐
select post '部门', max(salary) '薪资'
from emp group by post;

mysql> select post as '部门', max(salary) as'薪资'
-> from emp group by post;
+-----------------------------------+------------+
| 部门 | 薪资 |
+-----------------------------------+------------+
| NULL | NULL |
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 张江第一帅形象代言部门 | 7300.33 |
+-----------------------------------+------------+
5 rows in set (0.00 sec)

3.每个部门的最低工资
select post, min(salary) from emp group by post;

4.每个部门的平均工资
select post, avg(salary) from emp group by post;

5.每个部门的工资总和
select post, sum(salary) from emp group by post;

6.每个部门的员工个数
# count(): 括号中可以填任意非空值
select post, count(salary) from emp group by post;
select post, count(post_comment) from emp group by post;

查询岗位名以及各岗位内包含的员工个数
select post, count(id) from emp group by post;
查询公司内男员工和女员工的个数
select sex, count(*) from emp group by sex;

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
select sex, avg(salary) from emp group by sex;

.统计各部门年龄在30岁以上的员工平均工资:
# 步骤: 先找到表，再找年龄30岁以上，再根据部门分组，最后求平均薪资;
select post, avg(salary) from emp where age > 30 group by post;

总结: 聚合函数，必须跟在group by 后面(执行顺序);

# 聚合函数: 若没有group by 分组，默认将查出来的数据当做一个分组, 也能使用;
select max(salary) from emp; # group_concat(name): 可以将分组后的 所有名字获取并进行拼接
# 默认以, 拼接
select post, group_concat(name) from emp group by post;
# 指定以:拼接
select post, group_concat(name, ':') from emp group by post;
#
select post, group_concat('Name: ', name) from emp group by post;

select post, group_coucat(name) from emp group by post;

mysql> select post, group_concat(name) from emp group by post;
+-----------------------------------+------------------------------------------------+
| post | group_concat(name) |
+-----------------------------------+------------------------------------------------+
| NULL | o |
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| sale | 格格,星星,丁丁,丫丫,歪歪 |
| teacher | sean,大饼,jerry,owen,jason,kevin,egon |
| 张江第一帅形象代言部门 | tank |
+-----------------------------------+------------------------------------------------+
5 rows in set (0.00 sec)

# 给每个部门的员工名字前 + NB_
select post, group_concat('NB_', name) from emp group by post;
# 拼接部门员工名字+薪资
select post, group_concat(name, ":", salary) from emp group by post;

补充:concat

# 4.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法并用
select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp;

mysql> select concat('name: ', name) as '名字',concat('sal: ',salary) as '薪资' from emp;
+-----------------+-----------------+
| 名字 | 薪资 |
+-----------------+-----------------+
| name: tank | sal: 7300.33 |
| name: egon | sal: 1000000.31 |
| name: kevin | sal: 8300.00 |
| name: jason | sal: 3500.00 |
| name: owen | sal: 2100.00 |
| name: jerry | sal: 9000.00 |
| name: 大饼 | sal: 30000.00 |
| name: sean | sal: 10000.00 |
| name: 歪歪 | sal: 3000.13 |
| name: 丫丫 | sal: 2000.35 |
| name: 丁丁 | sal: 1000.37 |
| name: 星星 | sal: 3000.29 |
| name: 格格 | sal: 4000.33 |
| name: 张野 | sal: 10000.13 |
| name: 程咬金 | sal: 20000.00 |
| name: 程咬银 | sal: 19000.00 |
| name: 程咬铜 | sal: 18000.00 |
| name: 程咬铁 | sal: 17000.00 |
| name: o | NULL |
+-----------------+-----------------+
19 rows in set (0.00 sec)

3.having(过滤)

1.having与where语法一样，只不过having必需要在group by后使用;
2.where 不能使用聚合函数，但having可以;

- 书写顺序:
- select
- from
- where
- group by
- having

- 执行顺序:
- from
- where
- group by
- having
- select

1、统计各 部门 年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门;
select post, avg(salary) from emp where age > 30 group by post having avg(salary) > 10000;
mysql> select post, avg(salary) from emp where age > 30 group by post having avg(salary) > 10000;
+---------+---------------+
| post | avg(salary) |
+---------+---------------+
| teacher | 255450.077500 |
+---------+---------------+
1 row in set (0.00 sec)

4.distinct(去重)

# distinct: 去重

- 书写顺序:
- select
- distinct
- from
- where
- group by
- having

执行顺序:
- from
- where
- group by
- having
- select
- distinct

# 注意: 查询的字段值必须是重复的才有效，只要有一个字段值是不重复的就没有效果。

# 若所查字段内有不重复的字段记录就不会去重
select distinct id, post from emp;

mysql> select distinct id, post from emp;
+----+-----------------------------------+
| id | post |
+----+-----------------------------------+
| 1 | 张江第一帅形象代言部门 |
| 2 | teacher |
| 3 | teacher |
| 4 | teacher |
| 5 | teacher |
| 6 | teacher |
| 7 | teacher |
| 8 | teacher |
| 9 | sale |
| 10 | sale |
| 11 | sale |
| 12 | sale |
| 13 | sale |
| 14 | operation |
| 15 | operation |
| 16 | operation |
| 17 | operation |
| 18 | operation |
| 19 | NULL |
+----+-----------------------------------+
19 rows in set (0.00 sec)

select distinct post from emp;

mysql> select distinct post from emp;
+-----------------------------------+
| post |
+-----------------------------------+
| 张江第一帅形象代言部门 |
| teacher |
| sale |
| operation |
| NULL |
+-----------------------------------+
5 rows in set (0.00 sec)

5.order by(排序)

- 书写顺序:
- select
- from
- where
- group by
- having
- order by

执行顺序:
- from
- where
- group by
- having
- select
- order by # 通过select 查出来的数据再进行排序

# order by 默认升序
# asc升序
# desc降序

# 1、根据薪资进行升序
select name, salary from emp order by salary; # 默认升序

mysql> select name, salary from emp order by salary;
+-----------+------------+
| name | salary |
+-----------+------------+
| o | NULL |
| 丁丁 | 1000.37 |
| 丫丫 | 2000.35 |
| owen | 2100.00 |
| 歪歪 | 3000.13 |
| 星星 | 3000.29 |
| jason | 3500.00 |
| 格格 | 4000.33 |
| tank | 7300.33 |
| kevin | 8300.00 |
| jerry | 9000.00 |
| sean | 10000.00 |
| 张野 | 10000.13 |
| 程咬铁 | 17000.00 |
| 程咬铜 | 18000.00 |
| 程咬银 | 19000.00 |
| 程咬金 | 20000.00 |
| 大饼 | 30000.00 |
| egon | 1000000.31 |
+-----------+------------+
19 rows in set (0.00 sec)

2.select * from emp order by salary asc; # 指定升序

3.select name, salary from emp order by salary desc; # 指定降序

mysql> select name, salary from emp order by salary desc;
+-----------+------------+
| name | salary |
+-----------+------------+
| egon | 1000000.31 |
| 大饼 | 30000.00 |
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
| 张野 | 10000.13 |
| sean | 10000.00 |
| jerry | 9000.00 |
| kevin | 8300.00 |
| tank | 7300.33 |
| 格格 | 4000.33 |
| jason | 3500.00 |
| 星星 | 3000.29 |
| 歪歪 | 3000.13 |
| owen | 2100.00 |
| 丫丫 | 2000.35 |
| 丁丁 | 1000.37 |
| o | NULL |
+-----------+------------+
19 rows in set (0.00 sec)

# 先按照age升序，再按照salary降序
select age, salary from emp order by age asc, salary desc;

# 统计 各部门(分组) 年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行升序序
select post, avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary);

9.limit(限制查询记录的数量)

书写顺序:
- select
- from
- order by
- limit

执行顺序:
- from
- select
- order by
- limit

# 1、从第一条开始，获取4条记录;
select * from emp limit 4;

mysql> select * from emp limit 4;
+----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+
| 1 | tank | male | 17 | 2017-03-01 | 张江第一帅形象代言部门 | NULL | 7300.33 | 401 | 1 |
| 2 | egon | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
+----+-------+------+-----+------------+-----------------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)

# 2、limit可以有两个参数, 参数1:是限制的开始位置, 参数2:是从开始位置展示的条数;
select * from emp limit 0, 4;
select * from emp limit 4, 4;

mysql> select * from emp limit 4, 4;
+----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | 大饼 | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 8 | sean | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
+----+--------+--------+-----+------------+---------+--------------+----------+--------+-----------+
4 rows in set (0.00 sec)

# 3、查询工资最高的人的详细信息
select * from emp order by salary limit 1;

mysql> select * from emp order by salary limit 1;
+----+------+------+-----+------------+------+--------------+--------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+------+-----+------------+------+--------------+--------+--------+-----------+
| 19 | o | male | 28 | 0000-00-00 | NULL | NULL | NULL | NULL | NULL |
+----+------+------+-----+------------+------+--------------+--------+--------+-----------+
1 row in set (0.00 sec)

10.正则

# 在编程中，凡是看到reg开头的，基本上都是跟正则有关
# *: 代表 0 或 多个
select * from emp where name regexp '^程.*(金|银|铜|铁)$';

mysql> select * from emp where name regexp '^程.*(金|银|铜|铁)$';
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
4 rows in set (0.00 sec)三.多表查询

多表查询

-关联查询

-子查询

创建表与插入数据准备

#建表
create table dep2(
id int,
name varchar(20)
);

create table emp2(
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 dep2 values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into emp2(name,sex,age,dep_id) values
('tank','male',17,200),
('egon','female',48,201),
('kevin','male',38,201),
('jason','female',28,202),
('owen','male',18,200),
('sean','female',18,204);

mysql> select * from dep2;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+
4 rows in set (0.00 sec)

mysql> select * from emp2;
+----+-------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+-------+--------+------+--------+
| 1 | tank | male | 17 | 200 |
| 2 | egon | female | 48 | 201 |
| 3 | kevin | male | 38 | 201 |
| 4 | jason | female | 28 | 202 |
| 5 | owen | male | 18 | 200 |
| 6 | sean | female | 18 | 204 |
+----+-------+--------+------+--------+

# 指到了如何根据表关系对字段进行拆分，目的是为了更好的管理，表数据都存放在硬盘中，存不是目的，目的是为了取，所以我们将数据从硬盘读到内存中，接下来我们因应该将他们拼成一张表来查询更加合理;

# 注意: 将拆分的表，再拼接到一起进行查询, 可以通过一张表查另一张表的数据;

1.关联查询

# 左表的一条记录与右表的一条记录都对应一遍称之为 --> "笛卡尔积" PS: 百度科普
# 将所有的数据都对应了一遍，虽然不合理但是其中有合理的数据，现在我们需要做的就是找出合理的数据举例:通过之前准备的数据进行多表操作

1、查询 员工 以及所在 部门 的信息;
# 通过where 约束条件
select * from emp2, dep2 where emp2.dep_id = dep2.id;

2、查询 部门 为 技术部 的 员工 及 部门信息

select * from emp2, dep2 where emp2.dep_id = dep2.id and dep2.name = '技术';联表查询:
-内链接:只去两张表有对应关系的记录
select * from emp2 inner join dep2 on emp2.dep_id = dep2.id;

mysql> select * from emp2 inner join dep2 on emp2.dep_id = dep2.id;
+----+-------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-------+--------+------+--------+------+--------------+
| 1 | tank | male | 17 | 200 | 200 | 技术 |
| 2 | egon | female | 48 | 201 | 201 | 人力资源 |
| 3 | kevin | male | 38 | 201 | 201 | 人力资源 |
| 4 | jason | female | 28 | 202 | 202 | 销售 |
| 5 | owen | male | 18 | 200 | 200 | 技术 |
+----+-------+--------+------+--------+------+--------------+
5 rows in set (0.05 sec)

2、left join
# 2、左连接: 在内连接的基础上保留左表没有对应关系的记录(以null补全)
select * from emp2 left join dep2 on emp2.dep_id = dep2.id;

mysql> select * from emp2 left join dep2 on emp2.dep_id = dep2.id;
+----+-------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-------+--------+------+--------+------+--------------+
| 1 | tank | male | 17 | 200 | 200 | 技术 |
| 5 | owen | male | 18 | 200 | 200 | 技术 |
| 2 | egon | female | 48 | 201 | 201 | 人力资源 |
| 3 | kevin | male | 38 | 201 | 201 | 人力资源 |
| 4 | jason | female | 28 | 202 | 202 | 销售 |
| 6 | sean | female | 18 | 204 | NULL | NULL |
+----+-------+--------+------+--------+------+--------------+

# 3、右连接: 在内连接的基础上保留右表没有对应关系的记录
select * from emp2 right join dep2 on emp2.dep_id = dep2.id;

mysql> select * from emp2 right join dep2 on emp2.dep_id = dep2.id;
+------+-------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+-------+--------+------+--------+------+--------------+
| 1 | tank | male | 17 | 200 | 200 | 技术 |
| 2 | egon | female | 48 | 201 | 201 | 人力资源 |
| 3 | kevin | male | 38 | 201 | 201 | 人力资源 |
| 4 | jason | female | 28 | 202 | 202 | 销售 |
| 5 | owen | male | 18 | 200 | 200 | 技术 |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
+------+-------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

- 全连接: union
# 4、全连接: 在内连接的基础上 保留左、右表没有对应关系的记录
select * from emp2 left join dep2 on emp2.dep_id = dep2.id
union
select * from emp2 right join dep2 on emp2.dep_id = dep2.id;

2.子查询

# 子查询就是将一个查询语句的结果用括号括起来，当做另一个查询语句的条件去用

# 查询部门是技术或者人力资源的员工信息
'''
先获取技术部和人力资源的id号，再去员工表里根据前面的id筛选出符合要求的员工信息;
'''
select * from emp2 where dep_id in (select id from dep2 where name='技术' or name='人力资源');

# 2.每个部门最新入职的员工 思路：先查每个部门最新入职的员工，再按部门对应上联表查询
# 查第一张emp表
# 第四步
select t1.id, t1.name, t1.hire_date, t2.*
from
# 第二步
# 拼接了 t1（emp） 与 t2（各部门最新入职的员工数据虚拟表）表的数据

emp as t1
inner join

# 第一步
# 子查询获取emp表中的 部门名称与最新入职的时间字段值 生成一张虚拟表
(select post, max(hire_date) as max_date from emp group by post) as t2
on
# 若 t1.post 的值与 t2.post 条件成立，则拼接对应的记录;
t1.post = t2.post
# 第三步
where t1.hire_date = t2.max_date;

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