一、内连接（表1 inner join 表2 on）

内连接就是利用where子句对两种表形成的笛卡尔积进行筛选，之前博客写的查询都是内连接，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;
--或者
select 字段 from 表1 inner join 表2 on 连接条件 where 条件 and 条件;

1、显示SMITH的名字和部门名称

--两张数据表
mysql> select* from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.07 sec)

mysql> select* from dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.00 sec)

之前的写法：

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)

标准的内连接的写法：

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)

mysql> select ename,dname from emp inner join dept on emp.deptno=dept.deptno where ename='SMITH';
+-------+----------+
| ename | dname    |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)

二、外连接

外连接分为左外连接和右外连接。

1、左外连接（表名1 left join 表名2 on）

如果联合查询，左侧的表完全显示就是左外连接。

语法：

select 字段名 from 表名1 left join 表名2 on 连接条件;

整两张表：

-- 学生表
create table stu (id int, name varchar(30)); 
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
--成绩表
create table exam (id int, grade int);
insert into exam values(1, 56),(2,76),(11, 8);

1.1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

学生表和成绩表的id并不是一一对应的，如果这里用内连接，未在两个表中出现的id字段将不会被筛选出来：

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)

所以此处需要使用左外连接：

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)

2、右外连接（表名1 right join 表名2）

如果联合查询，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;

2.1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

mysql> select exam.id,name,grade from stu right join exam on stu.id=exam.id;
+------+------+-------+
| id   | name | grade |
+------+------+-------+
|    1 | jack |    56 |
|    2 | tom  |    76 |
|   11 | NULL |     8 |
+------+------+-------+
3 rows in set (0.00 sec)

2.2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

mysql> select * from emp right join dept on emp.deptno=dept.deptno order by emp.deptno asc;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
|   NULL | NULL   | NULL      | NULL | NULL                |    NULL |    NULL |   NULL |     40 | OPERATIONS | BOSTON   |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
15 rows in set (0.00 sec)

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