文章目录
- 一、内连接
-
- [1.1 显示SMITH的名字和部门名称](#1.1 显示SMITH的名字和部门名称)
- 二、外连接
-
- [2.1 左外连接](#2.1 左外连接)
-
- [2.1.1 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](#2.1.1 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)
- [2.2 右外连接](#2.2 右外连接)
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- [2.2.1 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](#2.2.1 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)
- [2.2.2 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](#2.2.2 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)
一、内连接
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，就是在from中把表连接号然后利用where条件筛选。
语法
sql
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件;1.1 显示SMITH的名字和部门名称
sql
# 要用到的两张表
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 (0.00 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)- 通过笛卡尔积筛选
sql
mysql> select ename, dname from emp t1, dept t2 where t1.deptno=t2.deptno and t1.ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)- 标准的内连接的写法
sql
mysql> select ename, dname from emp t1 inner join dept t2 on t1.deptno=t2.deptno;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| JONES | RESEARCH |
| MARTIN | SALES |
| BLAKE | SALES |
| CLARK | ACCOUNTING |
| SCOTT | RESEARCH |
| KING | ACCOUNTING |
| TURNER | SALES |
| ADAMS | RESEARCH |
| JAMES | SALES |
| FORD | RESEARCH |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)二、外连接
2.1 左外连接
如果联合查询，左侧的表完全显示就是左外连接。
语法：
sql
select 字段名 from 表名1 left join 表名2 on 连接条件;现在有两个表结构：
sql
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)2.1.1 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
可以看到这两张表并不是完全吻合的，如果这里用内连接，未在两个表中同时出现的id字段将不会被筛选出来：
sql
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)- 左外连接
sql
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)2.2 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：
sql
select 字段 from 表名1 right join 表名2 on 连接条件;2.2.1 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
sql
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)2.2.2 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
sql
mysql> select dname, ename, dept.deptno from emp right join dept on dept.deptno=emp.deptno order by dept.deptno ASC;
+------------+--------+--------+
| dname | ename | deptno |
+------------+--------+--------+
| ACCOUNTING | KING | 10 |
| ACCOUNTING | CLARK | 10 |
| ACCOUNTING | MILLER | 10 |
| RESEARCH | FORD | 20 |
| RESEARCH | JONES | 20 |
| RESEARCH | SMITH | 20 |
| RESEARCH | ADAMS | 20 |
| RESEARCH | SCOTT | 20 |
| SALES | BLAKE | 30 |
| SALES | WARD | 30 |
| SALES | TURNER | 30 |
| SALES | MARTIN | 30 |
| SALES | ALLEN | 30 |
| SALES | JAMES | 30 |
| OPERATIONS | NULL | 40 |
+------------+--------+--------+
15 rows in set (0.00 sec)