表的连接分为内连和外连
内连接
内连接实际上就是利用where子句对两种表形成的笛卡儿积进行筛选，我们前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询。
语法：
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；复合查询那一节的写法本质上就是内连接，如下：
select 字段 from 表1， 表2 where 筛选条件；例子：显示SMITH的名字和部门名称
cpp
--复合查询中的多表查询
mysql> select ename, dname from emp, dept where emp.deptno=dept.deptno and ename = 'SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.01 sec)
--内连接
mysql> select ename, dname from emp inner join dept on emp.deptno=dept.deptno and ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)外连接
外连接分为左外连接和右外连接
左外连接
如果复合查询，左侧的表完全显示我们就说是左外连接（当左边表和右边表没有匹配时，也会显示左边表的数据。）
语法：
select 字段名 from 表名1 left join 表名2 on 连接条件例子：
建两张表
cpp
mysql> create table stu (id int, name varchar(30));
Query OK, 0 rows affected (0.38 sec)
mysql> insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
Query OK, 4 rows affected (0.04 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> create table exam (id int, grade int);
Query OK, 0 rows affected (0.07 sec)
mysql> insert into exam values(1, 56),(2,76),(11, 8);
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from stu;
+------+------+
| id | name |
+------+------+
| 1 | jack |
| 2 | tom |
| 3 | kity |
| 4 | nono |
+------+------+
4 rows in set (0.00 sec)
mysql> select * from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 8 |
+------+-------+
3 rows in set (0.00 sec)查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来。
cpp
--左外连接
mysql> select * from stu left join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
| 3 | kity | NULL | NULL |
| 4 | nono | NULL | NULL |
+------+------+------+-------+
4 rows in set (0.00 sec)
--内连接
mysql> select * from stu inner join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
+------+------+------+-------+
2 rows in set (0.00 sec)右外连接
如果复合查询，右侧的表完全显示我们就说是右外连接。
语法：
select 字段 from 表名1 right join 表名2 on 连接条件；例子：对stu表和exam表联合查询，把所有的成绩都显示出来，即使这个成绩没有学生与它对应，也要显示出来
cpp
--右外连接
mysql> select * from stu right join exam on stu.id=exam.id;
+------+------+------+-------+
| id | name | id | grade |
+------+------+------+-------+
| 1 | jack | 1 | 56 |
| 2 | tom | 2 | 76 |
| NULL | NULL | 11 | 8 |
+------+------+------+-------+
3 rows in set (0.00 sec)
--左外连接也可实现这样的效果（这里没有交换列）
mysql> select * from exam left join stu on stu.id=exam.id;
+------+-------+------+------+
| id | grade | id | name |
+------+-------+------+------+
| 1 | 56 | 1 | jack |
| 2 | 76 | 2 | tom |
| 11 | 8 | NULL | NULL |
+------+-------+------+------+
3 rows in set (0.00 sec)例子：列出部门名称和这些部门的员工信息，同时列出没有员工的部门
--左外连接
mysql> select dept.deptno, dept.dname, emp.ename from dept left join emp on dept.deptno=emp.dep
tno;
+--------+------------+--------+
| deptno | dname | ename |
+--------+------------+--------+
| 10 | ACCOUNTING | MILLER |
| 10 | ACCOUNTING | KING |
| 10 | ACCOUNTING | CLARK |
| 20 | RESEARCH | FORD |
| 20 | RESEARCH | ADAMS |
| 20 | RESEARCH | SCOTT |
| 20 | RESEARCH | JONES |
| 20 | RESEARCH | SMITH |
| 30 | SALES | JAMES |
| 30 | SALES | TURNER |
| 30 | SALES | BLAKE |
| 30 | SALES | MARTIN |
| 30 | SALES | WARD |
| 30 | SALES | ALLEN |
| 40 | OPERATIONS | NULL |
+--------+------------+--------+
15 rows in set (0.00 sec)
--右外连接
mysql> select dept.deptno, dept.dname, emp.ename from emp right join dept on dept.deptno=emp.de
ptno;
+--------+------------+--------+
| deptno | dname | ename |
+--------+------------+--------+
| 10 | ACCOUNTING | MILLER |
| 10 | ACCOUNTING | KING |
| 10 | ACCOUNTING | CLARK |
| 20 | RESEARCH | FORD |
| 20 | RESEARCH | ADAMS |
| 20 | RESEARCH | SCOTT |
| 20 | RESEARCH | JONES |
| 20 | RESEARCH | SMITH |
| 30 | SALES | JAMES |
| 30 | SALES | TURNER |
| 30 | SALES | BLAKE |
| 30 | SALES | MARTIN |
| 30 | SALES | WARD |
| 30 | SALES | ALLEN |
| 40 | OPERATIONS | NULL |
+--------+------------+--------+
15 rows in set (0.00 sec)