文章目录
-
- [1. 表的内连和外连](#1. 表的内连和外连)
-
- [1.1 内连接](#1.1 内连接)
- [1.2 外连接](#1.2 外连接)
-
- [1.2.1 左外连接](#1.2.1 左外连接)
- [1.2.2 右外连接](#1.2.2 右外连接)
1. 表的内连和外连
1.1 内连接
内连接实际上就是利用 where 字句对两张表形成的笛卡尔积进行筛选，前面也有使用过，开发过程中使用也很常见
语法：
sql
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件- 显示 "SMITH" 的名字和部门名称
之前的写法
sql
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> 内连接写法
sql
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> 1.2 外连接
外连接分为左外连接和右外连接
1.2.1 左外连接
如果联合查询中，左侧的表完全显示，就称之为作左外连接
语法：
sql
select 字段名 from 表1 left join 表2 on 连接条件 and 其他条件案例：
创建新的两张表
sql
mysql> create table if not exists stu(id int primary key auto_increment,name varchar(20));
Query OK, 0 rows affected (0.02 sec)
mysql> insert into stu(name) values('Jack'),('Tom'),('Kity'),('Nano');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> create table if not exists exam(id int primary key auto_increment,grade int unsigned);
Query OK, 0 rows affected (0.02 sec)
mysql> insert into exam values(1,57),(2,77),(10,9);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from stu;
+----+------+
| id | name |
+----+------+
| 1 | Jack |
| 2 | Tom |
| 3 | Kity |
| 4 | Nano |
+----+------+
4 rows in set (0.00 sec)
mysql> select * from exam;
+----+-------+
| id | grade |
+----+-------+
| 1 | 57 |
| 2 | 77 |
| 10 | 9 |
+----+-------+
3 rows in set (0.00 sec)
mysql> - 查询所有学生的成绩，如果这个学生没有成绩，也要讲学生的个人信息显示出来
sql
mysql> select * from stu left join exam on stu.id = exam.id;
+----+------+------+-------+
| id | name | id | grade |
+----+------+------+-------+
| 1 | Jack | 1 | 57 |
| 2 | Tom | 2 | 77 |
| 3 | Kity | NULL | NULL |
| 4 | Nano | NULL | NULL |
+----+------+------+-------+
4 rows in set (0.00 sec)
mysql> 1.2.2 右外连接
如果联合查询中，右侧的表完全显示，就称之为作右外连接
语法：
sql
select 字段名 from 表1 right join 表2 on 连接条件 and 其他条件案例：
- 查询所有学生的成绩，如果这些学生没有成绩，也要显示学生的个人信息
sql
mysql> select * from stu right join exam on stu.id = exam.id;
+------+------+----+-------+
| id | name | id | grade |
+------+------+----+-------+
| 1 | Jack | 1 | 57 |
| 2 | Tom | 2 | 77 |
| NULL | NULL | 10 | 9 |
+------+------+----+-------+
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
使用右外连接：
sql
mysql> select dept.dname,emp.* from emp right join dept on emp.deptno = dept.deptno;
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| dname | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)
mysql> 使用左外连接
sql
mysql> select dept.dname,emp.* from dept left join emp on dept.deptno = emp.deptno;
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| dname | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)
mysql>