详解MySQL中表的内外连接
作者：蒋灵瑜的笔记本
这篇文章主要为大家介绍了MySQL中表的内外连接的相关知识，文中通过示例代码介绍的非常详细，具有一定的参考学习价值，需要的朋友们下面随着小编来一起学习学习吧
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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