8-MySQL_表的内连和外连

✨✨ 欢迎大家来到小伞的大讲堂✨✨养成好习惯先赞后看哦~所属专栏小伞的主页gitee:许星让 (xu-xingrang) - Gitee.com制作不易点个赞吧谢谢喵8.1 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 其他条件案例显示SMITH的名字和部门名称-- 用前面的写法 mysql select ename, dname from emp, dept where emp.deptnodept.deptno and enameSMITH; ----------------- | ename | dname | ----------------- | SMITH | RESEARCH | ----------------- 1 row in set (0.00 sec) -- 用标准的内连接写法 mysql select ename, dname from emp inner join dept on emp.deptnodept.deptno and enameSMITH; ----------------- | ename | dname | ----------------- | SMITH | RESEARCH | ----------------- 1 row in set (0.00 sec)8.2 外连接外连接分为左外连接和右外连接8.2.1 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-- 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);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mysql select * from stu left join exam on stu.idexam.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)8.2.2 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select * from stu right join exam on stu.idexam.id;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 d.dname, e.* from dept d left join emp e on d.deptnoe.deptno; -------------------------------------------------------------------------------------------- | dname | empno | ename | job | mgr | hiredate | sal | comm | deptno | -------------------------------------------------------------------------------------------- | RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | -------------------------------------------------------------------------------------------- 15 rows in set (0.00 sec)mysql select d.dname, e.* from emp e right join dept d on d.deptnoe.deptno; -------------------------------------------------------------------------------------------- | dname | empno | ename | job | mgr | hiredate | sal | comm | deptno | -------------------------------------------------------------------------------------------- | RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | -------------------------------------------------------------------------------------------- 15 rows in set (0.00 sec)