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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)
+```
\ No newline at end of file