我要评论表的连接分为内连和外连
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，我们前面文章的查询都是内连接，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
用之前的方法
mysql> select ename, dname from emp, dept where emp.deptno=dept.deptno and ename='smith'; +-------+----------+ | ename | dname | +-------+----------+ | smith | research | +-------+----------+ 1 row in set (0.00 sec)
用标准内连接
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)
两种方式混合使用，更好的区分笛卡尔积和筛选条件
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)
外连接
外连接分为左外连接和右外连接
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
前期准备：
mysql> create table stu (id int, name varchar(30));-- 学生表 query ok, 0 rows affected (0.03 sec) mysql> desc stu; +-------+-------------+------+-----+---------+-------+ | field | type | null | key | default | extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | yes | | null | | | name | varchar(30) | yes | | null | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono'); query ok, 4 rows affected (0.01 sec) records: 4 duplicates: 0 warnings: 0 mysql> select * from stu; +------+------+ | id | name | +------+------+ | 1 | jack | | 2 | tom | | 3 | kity | | 4 | nono | +------+------+ 4 rows in set (0.00 sec) mysql> create table exam (id int, grade int); -- 成绩表 query ok, 0 rows affected (0.02 sec) mysql> desc exam; +-------+---------+------+-----+---------+-------+ | field | type | null | key | default | extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | yes | | null | | | grade | int(11) | yes | | null | | +-------+---------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> insert into exam values(1, 56),(2,76),(11, 8); query ok, 3 rows affected (0.00 sec) records: 3 duplicates: 0 warnings: 0 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
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)
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
运用左外连接
mysql> select dept.deptno, dname, ename from dept left join emp on dept.deptno=emp.deptno order by dept.deptno asc; +--------+------------+--------+ | deptno | dname | ename | +--------+------------+--------+ | 10 | accounting | clark | | 10 | accounting | miller | | 10 | accounting | king | | 20 | research | jones | | 20 | research | smith | | 20 | research | adams | | 20 | research | scott | | 20 | research | ford | | 30 | sales | martin | | 30 | sales | allen | | 30 | sales | james | | 30 | sales | blake | | 30 | sales | ward | | 30 | sales | turner | | 40 | operations | null | +--------+------------+--------+ 15 rows in set (0.00 sec)
运用右外连接
mysql> select dept.deptno, dname, ename from emp right join dept on dept.deptno=emp.deptno order by dept.deptno asc; +--------+------------+--------+ | deptno | dname | ename | +--------+------------+--------+ | 10 | accounting | clark | | 10 | accounting | miller | | 10 | accounting | king | | 20 | research | jones | | 20 | research | smith | | 20 | research | adams | | 20 | research | scott | | 20 | research | ford | | 30 | sales | martin | | 30 | sales | allen | | 30 | sales | james | | 30 | sales | blake | | 30 | sales | ward | | 30 | sales | turner | | 40 | operations | null | +--------+------------+--------+ 15 rows in set (0.00 sec)
⚠️ 写在最后：以上内容是我在学习以后得一些总结和概括，如有错误或者需要补充的地方欢迎各位大佬评论或者私信我交流！！！
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