数据库基础-内外连接

1. 连接介绍

表的连接分为内连和外连。
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，之前记录的博客都是内连接。
外连接：外连接分为左外连接和右外连接，如果联合查询，左侧的表完全显示就说是左外连接。右侧的表完全显示就说是右外连接。

1.1 内连接

-之前的写法
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)


- 内连接的格式
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)

1.2 外连接

数据准备：

• 创建两张表：

-数据准备
mysql> create table stu(
    -> id int,
    -> name varchar(30));
Query OK, 0 rows affected (0.04 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> create table exam(
    -> id int,
    -> grade int);
Query OK, 0 rows affected (0.03 sec)

mysql> insert into exam values(1, 56),(2, 76),(11, 8);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

- 查看表
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)

1.2.1 左外连接

• 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

- 显示左表，然后连接右表（条件是on 后面的筛选条件），右表不足用NULL表示
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)

1.2.2 右外连接

• 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)

2.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

-右连接
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)



-左连接

mysql> select d.dname,e.* from emp e left join dept d
    -> on e.deptno=d.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 |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)