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2023-05-09 05:14| 来源: 网络整理| 查看: 265

MySQL-----表的内连和外连 原创

拾至名瑰 2023-04-29 07:13:55 ©著作权

文章标签 mysql 数据库 sql 内连接 表名 文章分类 Python 后端开发

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文章目录前言一、内连接二、外连接1. 左外连接2. 右外连接总结

前言

表的连接分为内连和外连.

提示：以下是本篇文章正文内容，下面案例可供参考

一、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尔积进行筛选,前面的查询都是内连接,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；

备注:前面学习的都是内连接.

案例:显示SMITH的名字和部门名称

-- 用前面的写法 mysql> select emp.ename,dept.dname from emp,dept where emp.deptno=dept.deptno and emp.ename='SMITH'; +-------+----------+ | ename | dname | +-------+----------+ | SMITH | RESEARCH | +-------+----------+ 1 row in set (0.00 sec) -- 用标准的内连接写法 mysql> select emp.ename,dept.dname from emp inner join dept on emp.deptno=dept.deptno and emp.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

如果联合查询,左侧的表完全显示我们就说是左外连接. 语法:

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

案例:

-- 建两张表 create table stu (id int, name varchar(30)); -- 学生表 insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono'); create table exam (id int, grade int); -- 成绩表 insert into exam values(1, 56),(2,76),(11, 8); 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-- 当左边表和右边表没有匹配时，也会显示左边表的数据 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

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

案例:

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 * 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)

练习：

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方法一： mysql> select * from dept left join emp on emp.deptno=dept.deptno; +--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+ | deptno | dname | loc | empno | ename | job | mgr | hiredate | sal | comm | deptno | +--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+ | 20 | RESEARCH | DALLAS | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 30 | SALES | CHICAGO | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | 30 | SALES | CHICAGO | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | 20 | RESEARCH | DALLAS | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 30 | SALES | CHICAGO | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | 30 | SALES | CHICAGO | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 10 | ACCOUNTING | NEW YORK | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | 20 | RESEARCH | DALLAS | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 10 | ACCOUNTING | NEW YORK | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 30 | SALES | CHICAGO | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | 20 | RESEARCH | DALLAS | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | 30 | SALES | CHICAGO | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | 20 | RESEARCH | DALLAS | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | 40 | OPERATIONS | BOSTON | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | +--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+ 15 rows in set (0.00 sec) 方法二： mysql> select * from emp right join dept on emp.deptno=dept.deptno; +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+ | empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc | +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+ | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 40 | OPERATIONS | BOSTON | +--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+ 15 rows in set (0.00 sec)总结

(本章完!!!)

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