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<h2><center>数据库查询</center></h2>
------
## 一：基本查询
### 1. 简介
- 单表查询
- 简单查询
- 通过条件查询
- 查询排序
- 限制查询记录数
- 使用集合函数查询
- 分组查询
- 使用正则表达式查询
### 2. 案例
创建案例所需表：company.employee1
```
雇员编号 id int
雇员姓名 name varchar(30)
雇员性别 sex enum
雇用时期 hire_date date
职位 post varchar(50)
职位描述 job_description varchar(100)
薪水 salary double(15,2)
办公室 office int
部门编号 dep_id int
```
```SQL
mysql> create table company.employee1(
-> id int primary key auto_increment not null,
-> name varchar(30) not null,
-> sex enum('m','f') default 'm' not null,
-> hire_date date not null,
-> hire_date varchar(50) not null,
-> job_description varchar(100),
-> salary double(15,2) not null,
-> office int,
-> dep_id int
-> );
Query OK, 0 rows affected (0.00 sec)
```
插入模拟数据：
```SQL
mysql> insert into company.employee1(name,sex,hire_date,post,job_description,salary,office,dep_id) values
-> ('jack','m','20180202','instructor','teach',5000,501,100),
-> ('tom','m','20180203','instructor','teach',5500,501,100),
-> ('robin','m','20180202','instructor','teach',8000,501,100),
-> ('alice','f','20180202','instructor','teach',7200,501,100),
-> ('','m','20180202','hr','hrcc',600,502,101),
-> ('harry','m','20180202','hr',NULL,6000,502,101),
-> ('emma','f','20180206','sale','salecc',20000,503,102),
-> ('christine','f','20180205','sale','salecc',2200,503,102),
-> ('zhuzhu','m','20180205','sale',NULL,2200,503,102),
-> ('gougou','m','20180205','sale','',2200,503,102);
Query OK, 10 rows affected (0.00 sec)
Records: 10 Duplicates: 0 Warnings: 0
```
语法格式：
​ select 字段名称,字段名称2...... from 表名 [条件]
#### 简单查询
```SQL
mysql> select * from employee1;
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
| 1 | jack | m | 2018-02-02 | teach | 5000.00 | 501 | 100 | instructor |
| 2 | tom | m | 2018-02-03 | teach | 5500.00 | 501 | 100 | instructor |
| 3 | robin | m | 2018-02-02 | teach | 8000.00 | 501 | 100 | instructor |
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
| 5 | | m | 2018-02-02 | hrcc | 600.00 | 502 | 101 | hr |
| 6 | harry | m | 2018-02-02 | NULL | 6000.00 | 502 | 101 | hr |
| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale |
| 8 | christine | f | 2018-02-05 | salecc | 2200.00 | 503 | 102 | sale |
| 9 | zhuzhu | m | 2018-02-05 | NULL | 2200.00 | 503 | 102 | sale |
| 10 | gougou | m | 2018-02-05 | | 2200.00 | 503 | 102 | sale |
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
10 rows in set (0.00 sec)
mysql> select name,salary,dep_id from employee1 where id <= 5;
+-------+---------+--------+
| name | salary | dep_id |
+-------+---------+--------+
| jack | 5000.00 | 100 |
| tom | 5500.00 | 100 |
| robin | 8000.00 | 100 |
| alice | 7200.00 | 100 |
| | 600.00 | 101 |
+-------+---------+--------+
5 rows in set (0.00 sec)
```
#### 避免重复
不能部分使用DISTINCT，通常仅用于某一字段
```SQL
mysql> select post from employee1;
+------------+
| post |
+------------+
| instructor |
| instructor |
| instructor |
| instructor |
| hr |
| hr |
| sale |
| sale |
| sale |
| sale |
+------------+
10 rows in set (0.00 sec)
mysql> select distinct post from employee1;
+------------+
| post |
+------------+
| instructor |
| hr |
| sale |
+------------+
3 rows in set (0.00 sec)
```
#### 四则运算查询
```SQL
mysql> select name,salary,salary*14 from employee1;
+-----------+----------+-----------+
| name | salary | salary*14 |
+-----------+----------+-----------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+-----------+
10 rows in set (0.00 sec)
mysql> select name,salary,salary*14 as Annual_salary from employee1;
+-----------+----------+---------------+
| name | salary | Annual_salary |
+-----------+----------+---------------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+---------------+
10 rows in set (0.00 sec)
mysql> select name,salary,salary*14 Annual_salary from employee1;
+-----------+----------+---------------+
| name | salary | Annual_salary |
+-----------+----------+---------------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+---------------+
10 rows in set (0.00 sec)
```
#### 定义显示格式
CONCAT() 函数用于连接字符串
```SQL
mysql> select concat(name,'s annual salary:',salary*14) as Annual_salary from employee1;
+-----------------------------------+
| Annual_salary |
+-----------------------------------+
| jacks annual salary:70000.00 |
| toms annual salary:77000.00 |
| robins annual salary:112000.00 |
| alices annual salary:100800.00 |
| s annual salary:8400.00 |
| harrys annual salary:84000.00 |
| emmas annual salary:280000.00 |
| christines annual salary:30800.00 |
| zhuzhus annual salary:30800.00 |
| gougous annual salary:30800.00 |
+-----------------------------------+
10 rows in set (0.00 sec)
```
#### 单条件查询
```SQL
mysql> select name,post from employee1 where post='hr';
+-------+------+
| name | post |
+-------+------+
| | hr |
| harry | hr |
+-------+------+
2 rows in set (0.00 sec)
```
#### 多条件查询
```SQL
mysql> select name,salary from employee1 where post='hr' and salary>10000;
Empty set (0.00 sec)
mysql> select * from employee1 where salary>5000 and salary<10000 or dep_id=102;
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
| 2 | tom | m | 2018-02-03 | teach | 5500.00 | 501 | 100 | instructor |
| 3 | robin | m | 2018-02-02 | teach | 8000.00 | 501 | 100 | instructor |
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
| 6 | harry | m | 2018-02-02 | NULL | 6000.00 | 502 | 101 | hr |
| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale |
| 8 | christine | f | 2018-02-05 | salecc | 2200.00 | 503 | 102 | sale |
| 9 | zhuzhu | m | 2018-02-05 | NULL | 2200.00 | 503 | 102 | sale |
| 10 | gougou | m | 2018-02-05 | | 2200.00 | 503 | 102 | sale |
+----+-----------+-----+------------+-----------------+----------+--------+--------+------------+
8 rows in set (0.00 sec)
```
#### 关键字
BETWEEN AND
```SQL
mysql> select name,salary from employee1 where salary between 5000 and 15000;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| tom | 5500.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
+-------+---------+
5 rows in set (0.00 sec)
mysql> select name,salary from employee1 where salary not between 5000 and 15000;
+-----------+----------+
| name | salary |
+-----------+----------+
| | 600.00 |
| emma | 20000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
+-----------+----------+
5 rows in set (0.00 sec)
```
IS NULL
```SQL
mysql> select name,job_description from employee1 where job_description is null;
+--------+-----------------+
| name | job_description |
+--------+-----------------+
| harry | NULL |
| zhuzhu | NULL |
+--------+-----------------+
2 rows in set (0.00 sec)
mysql> select name,job_description from employee1 where job_description is not null;
+-----------+-----------------+
| name | job_description |
+-----------+-----------------+
| jack | teach |
| tom | teach |
| robin | teach |
| alice | teach |
| | hrcc |
| emma | salecc |
| christine | salecc |
| gougou | |
+-----------+-----------------+
8 rows in set (0.00 sec)
mysql> select name,job_description from employee1 where job_description='';
+--------+-----------------+
| name | job_description |
+--------+-----------------+
| gougou | |
+--------+-----------------+
1 row in set (0.00 sec)
```
注意：（NULL说明）
- 等价于没有任何值、是未知数
- NULL与0、空字符串、空格都不同,NULL没有分配存储空间
- 对空值做加、减、乘、除等运算操作，结果仍为空
- 比较时使用关键字用“is null”和“is not null”
- 排序时比其他数据都小（索引默认是降序排列，小→大），所以NULL值总是排在最前
#### IN集合查询
```SQL
mysql> select name,salary from employee1 where salary=4000 or salary=5000 or salary=6000 or salary=9000;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| harry | 6000.00 |
+-------+---------+
2 rows in set (0.00 sec)
mysql> select name,salary from employee1 where salary in (4000,5000,6000,9000);
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| harry | 6000.00 |
+-------+---------+
2 rows in set (0.00 sec)
mysql> select name,salary from employee1 where salary not in (4000,5000,6000,9000);
+-----------+----------+
| name | salary |
+-----------+----------+
| tom | 5500.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| | 600.00 |
| emma | 20000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
+-----------+----------+
8 rows in set (0.00 sec)
```
#### 模糊查询
关键字LIKE
通配符%：所有字符
通配符_： 一个字符
```SQL
mysql> select * from employee1 where name like 'al%';
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
1 row in set (0.00 sec)
mysql> select * from employee1 where name like 'al___';
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
1 row in set (0.00 sec)
```
#### 排序查询
```SQL
mysql> select name,salary from employee1 order by salary; //升序
+-----------+----------+
| name | salary |
+-----------+----------+
| | 600.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
| jack | 5000.00 |
| tom | 5500.00 |
| harry | 6000.00 |
| alice | 7200.00 |
| robin | 8000.00 |
| emma | 20000.00 |
+-----------+----------+
10 rows in set (0.00 sec)
mysql> select name,salary from employee1 order by salary desc; //降序
+-----------+----------+
| name | salary |
+-----------+----------+
| emma | 20000.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
| tom | 5500.00 |
| jack | 5000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
| | 600.00 |
+-----------+----------+
10 rows in set (0.00 sec)
mysql> select name,salary from employee1 order by salary desc limit 3; //控制显示前3行
+-------+----------+
| name | salary |
+-------+----------+
| emma | 20000.00 |
| robin | 8000.00 |
| alice | 7200.00 |
+-------+----------+
3 rows in set (0.00 sec)
mysql> select name,salary from employee1 order by salary desc limit 1,3; //从序号1开始显示三行的内容
+-------+---------+
| name | salary |
+-------+---------+
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
+-------+---------+
3 rows in set (0.00 sec)
```
注意：
- ascending 美音 /ə'sɛndɪŋ/ 升序
- descending 美音 /dɪ'sɛndɪŋ/ 降序
#### 集合函数查询
count：可以查看共有多少条记录
```SQL
mysql> select count(*) from employee1;
+----------+
| count(*) |
+----------+
| 10 |
+----------+
1 row in set (0.00 sec)
mysql> select count(name) from employee1;
+-------------+
| count(name) |
+-------------+
| 10 |
+-------------+
1 row in set (0.01 sec)
```
max：查看最大值
```SQL
mysql> select max(salary) from employee1;
+-------------+
| max(salary) |
+-------------+
| 20000.00 |
+-------------+
1 row in set (0.00 sec)
```
min：查看最小值
```SQL
mysql> select min(salary) from employee1;
+-------------+
| min(salary) |
+-------------+
| 600.00 |
+-------------+
1 row in set (0.00 sec)
```
avg：查看平均值
```SQL
mysql> select avg(salary) from employee1;
+-------------+
| avg(salary) |
+-------------+
| 5890.000000 |
+-------------+
1 row in set (0.00 sec)
```
sum：求和
sale这个部门的总工资
```SQL
mysql> select concat("Total Department Wages:",sum(salary)) from employee1 where post='sale';
+-----------------------------------------------+
| concat("Total Department Wages:",sum(salary)) |
+-----------------------------------------------+
| Total Department Wages:26600.00 |
+-----------------------------------------------+
1 row in set (0.00 sec)
```
获取薪水最高的这个人的详细信息
```SQL
mysql> select * from employee1 where salary=(select max(salary) from employee1);
+----+------+-----+------------+-----------------+----------+--------+--------+------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+------+-----+------------+-----------------+----------+--------+--------+------+
| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale |
+----+------+-----+------------+-----------------+----------+--------+--------+------+
1 row in set (0.00 sec)
```
#### 分组查询
GROUP BY和GROUP_CONCAT()函数一起使用
获取部门ID相同的员工并把名字拼接到一起
```SQL
mysql> select dep_id,group_concat(name) from employee1 group by dep_id;
+--------+------------------------------+
| dep_id | group_concat(name) |
+--------+------------------------------+
| 100 | jack,tom,robin,alice |
| 101 | ,harry |
| 102 | emma,christine,zhuzhu,gougou |
+--------+------------------------------+
3 rows in set (0.00 sec)
```
GROUP BY和集合函数一起使用
获取部门最高薪资
```SQL
mysql> select post,max(salary) from employee1 group by post;
+------------+-------------+
| post | max(salary) |
+------------+-------------+
| hr | 6000.00 |
| instructor | 8000.00 |
| sale | 20000.00 |
+------------+-------------+
3 rows in set (0.00 sec)
```
#### 正则查询
```SQL
以什么开头
mysql> select * from employee1 where name regexp '^ali';
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
1 row in set (0.00 sec)
以什么开头
mysql> select * from employee1 where name regexp 'ce$';
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor |
+----+-------+-----+------------+-----------------+---------+--------+--------+------------+
1 row in set (0.00 sec)
连续出现n次
mysql> select * from employee1 where name regexp 'm{2}';
+----+------+-----+------------+-----------------+----------+--------+--------+------+
| id | name | sex | hire_date | job_description | salary | office | dep_id | post |
+----+------+-----+------------+-----------------+----------+--------+--------+------+
| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale |
+----+------+-----+------------+-----------------+----------+--------+--------+------+
1 row in set (0.00 sec)
```
## 二：多表联合查询
### 1.数据准备
**表：company.employee2**
创建表：
```SQL
mysql> create table employee2(
-> emp_id int auto_increment primary key not null,
-> emp_name varchar(50),
-> age int,
-> dept_id int
-> );
Query OK, 0 rows affected (0.00 sec)
```
查看表结构：
```SQL
mysql> desc employee2;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| emp_id | int(11) | NO | PRI | NULL | auto_increment |
| emp_name | varchar(50) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| dept_id | int(11) | YES | | NULL | |
+----------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
```
插入模拟数据：
```SQL
mysql> insert into employee2(emp_name,age,dept_id) values
-> ('',19,200),
-> ('tom',26,201),
-> ('jack',30,201),
-> ('alice',24,202),
-> ('robin',40,200),
-> ('wxin',16,200),
-> ('natasha',28,204);
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
```
查看数据：
```SQL
mysql> select * from employee2;
+--------+----------+------+---------+
| emp_id | emp_name | age | dept_id |
+--------+----------+------+---------+
| 1 | | 19 | 200 |
| 2 | tom | 26 | 201 |
| 3 | jack | 30 | 201 |
| 4 | alice | 24 | 202 |
| 5 | robin | 40 | 200 |
| 6 | wxin | 16 | 200 |
| 7 | natasha | 28 | 204 |
+--------+----------+------+---------+
7 rows in set (0.00 sec)
```
**表：company.department2**
创建表：
```SQL
mysql> create table department2(
-> dept_id int,
-> dept_name varchar(100)
-> );
Query OK, 0 rows affected (0.01 sec)
```
查看表结构：
```SQL
mysql> desc department2;
+-----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| dept_id | int(11) | YES | | NULL | |
| dept_name | varchar(100) | YES | | NULL | |
+-----------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
```
模拟插入数据：
```SQL
mysql> insert into department2 values
-> (200,'hr'),
-> (201,'it'),
-> (202,'sale'),
-> (203,'fd');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
```
查看数据：
```SQL
sql> select * from department2;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 200 | hr |
| 201 | it |
| 202 | sale |
| 203 | fd |
+---------+-----------+
4 rows in set (0.00 sec)
```
### 2. 多表的连接查询
- 交叉连接：生成笛卡尔积，它不使用任何匹配条件；交叉联接返回左表中的所有行，左表中的每一行与右表中的所有行组合
- 内连接：只连接匹配的行
- 外连接：
1. 左连接：会显示左边表内所有的值，不论在右边表内匹不匹配
2. 右连接：会显示右边表内所有的值，不论在左边表内匹不匹配
- 全外连接：包含左、右两个表的全部行
**交叉连接**
```SQL
mysql> select employee2.emp_name,employee2.age,employee2.dept_id,department2.dept_name from employee2,department2;
+----------+------+---------+-----------+
| emp_name | age | dept_id | dept_name |
+----------+------+---------+-----------+
| | 19 | 200 | hr |
| | 19 | 200 | it |
| | 19 | 200 | sale |
| | 19 | 200 | fd |
| tom | 26 | 201 | hr |
| tom | 26 | 201 | it |
| tom | 26 | 201 | sale |
| tom | 26 | 201 | fd |
| jack | 30 | 201 | hr |
| jack | 30 | 201 | it |
| jack | 30 | 201 | sale |
| jack | 30 | 201 | fd |
| alice | 24 | 202 | hr |
| alice | 24 | 202 | it |
| alice | 24 | 202 | sale |
| alice | 24 | 202 | fd |
| robin | 40 | 200 | hr |
| robin | 40 | 200 | it |
| robin | 40 | 200 | sale |
| robin | 40 | 200 | fd |
| wxin | 16 | 200 | hr |
| wxin | 16 | 200 | it |
| wxin | 16 | 200 | sale |
| wxin | 16 | 200 | fd |
| natasha | 28 | 204 | hr |
| natasha | 28 | 204 | it |
| natasha | 28 | 204 | sale |
| natasha | 28 | 204 | fd |
+----------+------+---------+-----------+
28 rows in set (0.00 sec)
```
**内连接**
获取有部门的员工 (部门表中没有natasha所在的部门)
```SQL
mysql> select employee2.emp_name,employee2.age,employee2.dept_id,department2.dept_name from employee2,department2 where employee2.dept_id=department2.dept_id;
+----------+------+---------+-----------+
| emp_name | age | dept_id | dept_name |
+----------+------+---------+-----------+
| | 19 | 200 | hr |
| tom | 26 | 201 | it |
| jack | 30 | 201 | it |
| alice | 24 | 202 | sale |
| robin | 40 | 200 | hr |
| wxin | 16 | 200 | hr |
+----------+------+---------+-----------+
6 rows in set (0.00 sec)
mysql> select employee2.emp_name,department2.dept_name from employee2 inner join department2 on employee2.dept_id=department2.dept_id;
+----------+-----------+
| emp_name | dept_name |
+----------+-----------+
| | hr |
| tom | it |
| jack | it |
| alice | sale |
| robin | hr |
| wxin | hr |
+----------+-----------+
6 rows in set (0.00 sec)
```
**外连接**
语法：
​ SELECT 字段列表 FROM 表1 LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段;
注意：
​ 先用谁谁就是左
**左连接 left join**
```SQL
找出所有员工及所属的部门，包括没有部门的员工
mysql> select emp_id,emp_name,dept_name from employee2 left join department2 on employee2.dept_id=department2.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 5 | robin | hr |
| 6 | wxin | hr |
| 2 | tom | it |
| 3 | jack | it |
| 4 | alice | sale |
| 7 | natasha | NULL |
+--------+----------+-----------+
7 rows in set (0.00 sec)
```
**右连接 right join**
```SQL
找出所有部门包含的员工，包括空部门
mysql> select emp_id,emp_name,dept_name from employee2 right join department2 on employee2.dept_id=department2.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 2 | tom | it |
| 3 | jack | it |
| 4 | alice | sale |
| 5 | robin | hr |
| 6 | wxin | hr |
| NULL | NULL | fd |
+--------+----------+-----------+
7 rows in set (0.00 sec)
```
**全外连接**
```
mysql> select * from employee2 full join department2;
+--------+----------+------+---------+---------+-----------+
| emp_id | emp_name | age | dept_id | dept_id | dept_name |
+--------+----------+------+---------+---------+-----------+
| 1 | | 19 | 200 | 200 | hr |
| 1 | | 19 | 200 | 201 | it |
| 1 | | 19 | 200 | 202 | sale |
| 1 | | 19 | 200 | 203 | fd |
| 2 | tom | 26 | 201 | 200 | hr |
| 2 | tom | 26 | 201 | 201 | it |
| 2 | tom | 26 | 201 | 202 | sale |
| 2 | tom | 26 | 201 | 203 | fd |
| 3 | jack | 30 | 201 | 200 | hr |
| 3 | jack | 30 | 201 | 201 | it |
| 3 | jack | 30 | 201 | 202 | sale |
| 3 | jack | 30 | 201 | 203 | fd |
| 4 | alice | 24 | 202 | 200 | hr |
| 4 | alice | 24 | 202 | 201 | it |
| 4 | alice | 24 | 202 | 202 | sale |
| 4 | alice | 24 | 202 | 203 | fd |
| 5 | robin | 40 | 200 | 200 | hr |
| 5 | robin | 40 | 200 | 201 | it |
| 5 | robin | 40 | 200 | 202 | sale |
| 5 | robin | 40 | 200 | 203 | fd |
| 6 | wxin | 16 | 200 | 200 | hr |
| 6 | wxin | 16 | 200 | 201 | it |
| 6 | wxin | 16 | 200 | 202 | sale |
| 6 | wxin | 16 | 200 | 203 | fd |
| 7 | natasha | 28 | 204 | 200 | hr |
| 7 | natasha | 28 | 204 | 201 | it |
| 7 | natasha | 28 | 204 | 202 | sale |
| 7 | natasha | 28 | 204 | 203 | fd |
+--------+----------+------+---------+---------+-----------+
28 rows in set (0.00 sec)
```
### 3. 复合条件连接查询
#### 案例一
​ 找出公司所有部门中年龄大于25岁的员工
​ 以内连接的方式查询employee6和department6表，并且employee6表中的age字段值必须大于25
```SQL
mysql> select emp_id,emp_name,dept_name from employee2,department2 where employee2.dept_id=department2.dept_id and age>25;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 5 | robin | hr |
| 2 | tom | it |
| 3 | jack | it |
+--------+----------+-----------+
3 rows in set (0.00 sec)
```
#### 案例二
​ 以内连接的方式查询employee6和department6表，并且以age字段的升序方式显示
```SQL
mysql> select emp_id,emp_name,dept_name from employee2,department2 where employee2.dept_id=department2.dept_id order by age asc;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 6 | wxin | hr |
| 1 | | hr |
| 4 | alice | sale |
| 2 | tom | it |
| 3 | jack | it |
| 5 | robin | hr |
+--------+----------+-----------+
6 rows in set (0.00 sec)
```
### 4. 子查询
​ 子查询是将一个查询语句嵌套在另一个查询语句中
​ 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
​ 子查询中可以包含：IN、NOT IN等关键字；还可以包含比较运算符：= 、 !=、> 、<等
#### 案例一
​ 带IN关键字的子查询；查询employee表，但dept_id必须在department表中出现过
```SQL
ysql> select * from employee2 where dept_id in (select dept_id from department2);
+--------+----------+------+---------+
| emp_id | emp_name | age | dept_id |
+--------+----------+------+---------+
| 1 | | 19 | 200 |
| 2 | tom | 26 | 201 |
| 3 | jack | 30 | 201 |
| 4 | alice | 24 | 202 |
| 5 | robin | 40 | 200 |
| 6 | wxin | 16 | 200 |
+--------+----------+------+---------+
6 rows in set (0.00 sec)
```
#### 案例二
​ 带比较运算符的子查询；查询年龄大于等于25岁员工所在部门（查询老龄化的部门）
```SQL
mysql> select dept_id,dept_name from department2 where dept_id in (select distinct dept_id from employee2 where age>=25);
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 201 | it |
| 200 | hr |
+---------+-----------+
2 rows in set (0.00 sec)
```