MySQL内外连接

文章目录

MySQL内外连接

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尔积进行筛选，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：
mysql 复制代码
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件;
注意：前面学的都是内连接。

案例：显示SMITH的名字和部门名称

用前面的写法：
mysql 复制代码
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)

mysql>
用标准的内连接写法：
mysql 复制代码
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>

2、外连接

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：
mysql 复制代码
select 字段名 from 表名1 left join 表名2 on 连接条件;
案例：

先建立两张表
mysql 复制代码
create table stu (id int, name varchar(30)); -- 学生表
create table exam (id int, grade int); -- 成绩表
插入数据
mysql 复制代码
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
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 复制代码
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)

mysql>

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：

mysql 复制代码

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 复制代码

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)

mysql>

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：
使用左外连接：

mysql 复制代码

mysql> select d.dname,e.* from DEPT d left join EMP e on d.deptno=e.deptno;
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
| dname      | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM   | DEPTNO |
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
| RESEARCH   |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |   NULL |     20 |
| SALES      |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 | 300.00 |     30 |
| SALES      |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 | 500.00 |     30 |
| RESEARCH   |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |   NULL |     20 |
| SALES      |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |   NULL |     30 |
| ACCOUNTING |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |   NULL |     10 |
| RESEARCH   |  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 | 3000.00 |   NULL |     20 |
| ACCOUNTING |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |   NULL |     10 |
| SALES      |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |   0.00 |     30 |
| RESEARCH   |  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 | 1100.00 |   NULL |     20 |
| SALES      |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |   NULL |     30 |
| RESEARCH   |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |   NULL |     20 |
| ACCOUNTING |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |   NULL |     10 |
| OPERATIONS |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |   NULL |   NULL |
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
14 rows in set (0.00 sec)

mysql>

使用右外连接：

mysql 复制代码

mysql> select d.dname,e.* from EMP e right join DEPT d on d.deptno=e.deptno;
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
| dname      | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM   | DEPTNO |
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
| RESEARCH   |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |   NULL |     20 |
| SALES      |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 | 300.00 |     30 |
| SALES      |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 | 500.00 |     30 |
| RESEARCH   |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |   NULL |     20 |
| SALES      |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |   NULL |     30 |
| ACCOUNTING |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |   NULL |     10 |
| RESEARCH   |  7788 | SCOTT  | ANALYST   | 7566 | 1982-12-09 | 3000.00 |   NULL |     20 |
| ACCOUNTING |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |   NULL |     10 |
| SALES      |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |   0.00 |     30 |
| RESEARCH   |  7876 | ADAMS  | CLERK     | 7788 | 1983-01-12 | 1100.00 |   NULL |     20 |
| SALES      |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |   NULL |     30 |
| RESEARCH   |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |   NULL |     20 |
| ACCOUNTING |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |   NULL |     10 |
| OPERATIONS |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |   NULL |   NULL |
+------------+-------+--------+-----------+------+------------+---------+--------+--------+
14 rows in set (0.00 sec)

mysql>

OKOK，MySQL内外连接就到这里，如果你对Linux和C++也感兴趣的话，可以看看我的主页哦。下面是我的github主页，里面记录了我的学习代码和leetcode的一些题的题解，有兴趣的可以看看。

Xpccccc的github主页