详解MySQL中表的内外连接

目录一、内连接（表1 inner join 表2 on）1、显示SMITH的名字和部门名称二、外连接1、左外连接（表名1 left join 表名2 on）2、右外连接（表名1 right join 表名2）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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