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第三章：数据库查询.md
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<h1><center>数据库查询</center></h1>
**作者：行癫（盗版必究）**
------
## 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.employee5
```shell
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
```
```shell
MySQL [(none)]> CREATE TABLE company.employee5(
id int primary key AUTO_INCREMENT not null,
name varchar(30) not null,
sex enum('male','female') default 'male' not null,
hire_date date not null,
post varchar(50) not null,
job_description varchar(100),
salary double(15,2) not null,
office int,
dep_id int
);
```
插入模拟数据：
```shell
MySQL [(none)]> insert into company.employee5(name,sex,hire_date,post,job_description,salary,office,dep_id) values
('jack','male','20180202','instructor','teach',5000,501,100),
('tom','male','20180203','instructor','teach',5500,501,100),
('robin','male','20180202','instructor','teach',8000,501,100),
('alice','female','20180202','instructor','teach',7200,501,100),
('','male','20180202','hr','hrcc',600,502,101),
('harry','male','20180202','hr',NULL,6000,502,101),
('emma','female','20180206','sale','salecc',20000,503,102),
('christine','female','20180205','sale','salecc',2200,503,102),
('zhuzhu','male','20180205','sale',NULL,2200,503,102),
('gougou','male','20180205','sale','',2200,503,102);
```
语法格式：
​ select 字段名称,字段名称2...... from 表名 [条件]
##### a.简单查询
```shell
MySQL [company]> select * from employee5;
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| 1 | jack | male | 2018-02-02 | instructor | teach | 5000.00 | 501 | 100 |
| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
| 5 | | male | 2018-02-02 | hr | hrcc | 600.00 | 502 | 101 |
| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
10 rows in set (0.00 sec)
MySQL [company]> select name, salary, dep_id from employee5 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)
```
##### b.避免重复
​ 不能部分使用DISTINCT，通常仅用于某一字段
```shell
MySQL [company]> SELECT post FROM employee5;
+------------+
| post |
+------------+
| instructor |
| instructor |
| instructor |
| instructor |
| hr |
| hr |
| sale |
| sale |
| sale |
| sale |
+------------+
10 rows in set (0.00 sec)
MySQL [company]> SELECT distinct post FROM employee5;
+------------+
| post |
+------------+
| instructor |
| hr |
| sale |
+------------+
3 rows in set (0.00 sec)
```
##### c.四则运算查询
```shell
MySQL [company]> SELECT name, salary, salary*14 FROM employee5;
+-----------+----------+-----------+
| 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 (0.01 sec)
MySQL [company]> SELECT name, salary, salary*14 AS Annual_salary FROM employee5;
+-----------+----------+---------------+
| 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 [company]> SELECT name, salary, salary*14 Annual_salary FROM employee5;
+-----------+----------+---------------+
| 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)
```
##### d.定义显示格式
​ CONCAT() 函数用于连接字符串
```shell
MySQL [company]> SELECT concat(name, 's annual salary: ', salary*14) AS Annual_salary FROM employee5;
+------------------------------------+
| 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)
```
##### e.单条件查询
```shell
MySQL [company]> SELECT name,post FROM employee5 WHERE post='hr';
+-------+------+
| name | post |
+-------+------+
| | hr |
| harry | hr |
+-------+------+
2 rows in set (0.00 sec)
```
##### f.多条件查询
```shell
MySQL [company]> SELECT name,salary FROM employee5 WHERE post='hr' AND salary>10000;
Empty set (0.00 sec)
MySQL [company]> select * from employee5 where salary>5000 and salary<10000 or dep_id=102;
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
8 rows in set (0.00 sec)
```
##### g.关键字
​ BETWEEN AND
```shell
MySQL [company]> SELECT name,salary FROM employee5 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 [company]> SELECT name,salary FROM employee5 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
```shell
MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NULL;
+--------+-----------------+
| name | job_description |
+--------+-----------------+
| harry | NULL |
| zhuzhu | NULL |
+--------+-----------------+
2 rows in set (0.00 sec)
MySQL [company]> SELECT name,job_description FROM employee5 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 [company]> SELECT name,job_description FROM employee5 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
```shell
MySQL [company]> SELECT name, salary FROM employee5 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 [company]> SELECT name, salary FROM employee5 WHERE salary IN (4000,5000,6000,9000) ;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| harry | 6000.00 |
+-------+---------+
2 rows in set (0.00 sec)
MySQL [company]> SELECT name, salary FROM employee5 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 (0.01 sec)
```
##### h.模糊查询
​ 关键字LIKE
​ 通配符%：所有字符
​ 通配符_： 一个字符
```shell
MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al%';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al___';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
```
##### i.排序查询
```shell
MySQL [company]> select name,salary from employee5 order by salary;
+-----------+----------+
| 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 (0.01 sec)
MySQL [company]> select name,salary from employee5 order by salary desc;
+-----------+----------+
| 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 |
+-----------+----------+
MySQL [company]> select name,salary from employee5 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 [company]> select name,salary from employee5 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
##### j.集合函数查询
​ count：可以查看共有多少条记录
```shell
MySQL [company]> select count(*) from employee5;
+----------+
| count(*) |
+----------+
| 10 |
+----------+
1 row in set (0.00 sec)
MySQL [company]> select count(name) from employee5;
+-------------+
| count(name) |
+-------------+
| 10 |
+-------------+
1 row in set (0.00 sec)
```
​ max：查看最大值
```shell
MySQL [company]> select max(salary) from employee5;
+-------------+
| max(salary) |
+-------------+
| 20000.00 |
+-------------+
1 row in set (0.00 sec)
```
​ min：查看最小值
```shell
MySQL [company]> select min(salary) from employee5;
+-------------+
| min(salary) |
+-------------+
| 600.00 |
+-------------+
1 row in set (0.00 sec)
```
​ avg：查看平均值
```shell
MySQL [company]> select avg(salary) from employee5;
+-------------+
| avg(salary) |
+-------------+
| 5890.000000 |
+-------------+
1 row in set (0.00 sec)
```
​ sum：求和
​ sale这个部门的总工资
```shell
MySQL [company]> select concat("Total Department Wages：",sum(salary)) from employee5 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
```shell
MySQL [company]> select * from employee5 where salary = (select max(salary) from employee5);
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
1 row in set (0.00 sec)
```
##### k.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
```shell
MySQL [company]> SELECT dep_id,GROUP_CONCAT(name) FROM employee5 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 (0.01 sec)
```
​ GROUP BY和集合函数一起使用
​ 获取部门最高薪资
```shell
MySQL [company]> SELECT post,max(salary) FROM employee5 GROUP BY post;
+------------+-------------+
| post | max(salary) |
+------------+-------------+
| hr | 6000.00 |
| instructor | 8000.00 |
| sale | 20000.00 |
+------------+-------------+
3 rows in set (0.00 sec)
```
##### l.正则查询
```shell
以什么开头
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP '^ali';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
以什么结尾
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'ce$';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.01 sec)
连续出现n次
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'm{2}';
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
1 row in set (0.00 sec)
```
## 二：多表联合查询
【扩展了解】
#### 1.数据准备
##### 表：company.employee6
创建表：
```shell
MySQL [company]> create table employee6(
emp_id int auto_increment primary key not null,
emp_name varchar(50),
age int,
dept_id int);
Query OK, 0 rows affected (0.65 sec)
```
查看表结构：
```shell
MySQL [company]> desc employee6;
+----------+-------------+------+-----+---------+----------------+
| 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：
```shell
MySQL [company]> insert into employee6(emp_name,age,dept_id) values
('',19,200),
('tom',26,201),
('jack',30,201),
('alice',24,202),
('robin',40,200),
('xingdian',16,200),
('natasha',28,204);
```
查看数据：
```shell
MySQL [company]> select * from employee6;
+--------+----------+------+---------+
| 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 | xingdian | 16 | 200 |
| 7 | natasha | 28 | 204 |
+--------+----------+------+---------+
7 rows in set (0.00 sec)
```
##### 表：company.department6
创建表：
```shell
MySQL [company]> create table department6(
dept_id int,
dept_name varchar(100)
);
Query OK, 0 rows affected (0.33 sec)
```
查看表结构：
```shell
MySQL [company]> desc department6;
+-----------+--------------+------+-----+---------+-------+
| 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：
```shell
MySQL [company]> insert into department6 values
(200,'hr'),
(201,'it'),
(202,'sale'),
(203,'fd');
```
查看数据：
```shell
MySQL [company]> select * from department6;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 200 | hr |
| 201 | it |
| 202 | sale |
| 203 | fd |
+---------+-----------+
4 rows in set (0.01 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
​ 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
​ 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
```shell
MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6;
+----------+------+---------+-----------+
| 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 |
| xingdian | 16 | 200 | hr |
| xingdian | 16 | 200 | it |
| xingdian | 16 | 200 | sale |
| xingdian | 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)
```shell
MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6 where employee6.dept_id=department6.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 |
| xingdian | 16 | 200 | hr |
+----------+------+---------+-----------+
6 rows in set (0.00 sec)
MySQL [company]> select employee6.emp_name,department6.dept_name from employee6 inner join department6 on employee6.dept_id=department6.dept_id;
+----------+-----------+
| emp_name | dept_name |
+----------+-----------+
| | hr |
| tom | it |
| jack | it |
| alice | sale |
| robin | hr |
| xingdian | hr |
+----------+-----------+
6 rows in set (0.01 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
```shell
找出所有员工及所属的部门，包括没有部门的员工
MySQL [company]> select emp_id,emp_name,dept_name from employee6 left join department6 on employee6.dept_id = department6.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 5 | robin | hr |
| 6 | xingdian | 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
```
找出所有部门包含的员工，包括空部门
MySQL [company]> select emp_id,emp_name,dept_name from employee6 right join department6 on employee6.dept_id = department6.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 2 | tom | it |
| 3 | jack | it |
| 4 | alice | sale |
| 5 | robin | hr |
| 6 | xingdian | hr |
| NULL | NULL | fd |
+--------+----------+-----------+
7 rows in set (0.00 sec)
```
##### 全外连接
```shell
MySQL [company]> select * from employee6 full join department6;
+--------+----------+------+---------+---------+-----------+
| 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 | xingdian | 16 | 200 | 200 | hr |
| 6 | xingdian | 16 | 200 | 201 | it |
| 6 | xingdian | 16 | 200 | 202 | sale |
| 6 | xingdian | 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
```shell
MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id AND age > 25;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 5 | robin | hr |
| 2 | tom | it |
| 3 | jack | it |
+--------+----------+-----------+
3 rows in set (0.01 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
```shell
MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id ORDER BY age asc;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 6 | xingdian | hr |
| 1 | | hr |
| 4 | alice | sale |
| 2 | tom | it |
| 3 | jack | it |
| 5 | robin | hr |
+--------+----------+-----------+
```
#### 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
```shell
MySQL [company]> select * from employee6 WHERE dept_id IN (select dept_id FROM department6);
+--------+----------+------+---------+
| 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 | xingdian | 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）
```shell
MySQL [company]> select dept_id,dept_name FROM department6 WHERE dept_id IN (SELECT DISTINCT dept_id FROM employee6 WHERE age >=25);
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 201 | it |
| 200 | hr |
+---------+-----------+
2 rows in set (0.00 sec)
```