https://www.bilibili.com/video/BV1hg411V7Bm/?p=93
并、交、差
结构相同
列：一列（数据元素）包含了相同的数据，例如邮政编码的数据。
行：一行（=元组，或记录）是一组相关的数据，例如一条用户订阅的数据。
| 关系S1
|
| Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
| No0003
|
Candy
|
IS
|
| No0004
|
Jam
|
IS
|
|
| 关系S2
|
| Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
| No0008
|
Katter
|
IS
|
| No0021
|
Tom
|
IS
|
|
并
| S1∪S2（并）
|
| Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
| No0003
|
Candy
|
IS
|
| No0004
|
Jam
|
IS
|
| No0008
|
Katter
|
IS
|
| No0021
|
Tom
|
IS
|
交
| S1∩S2（交）
|
| Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
差
| S1-S2（差）
|
| Sno
|
Sname
|
Sdept
|
| No0003
|
Candy
|
IS
|
| No0004
|
Jam
|
IS
|
笛卡尔积、投影和选择
| S1×S2（笛卡尔积）
|
| Sno
|
Sname
|
Sdept
|
Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
No0001
|
Mary
|
IS
|
| No0001
|
Mary
|
IS
|
No0008
|
Katter
|
IS
|
| No0001
|
Mary
|
IS
|
No0021
|
Tom
|
IS
|
| No0003
|
Candy
|
IS
|
No0001
|
Mary
|
IS
|
| No0003
|
Candy
|
IS
|
No0008
|
Katter
|
IS
|
| No0003
|
Candy
|
IS
|
No0021
|
Tom
|
IS
|
| No0004
|
Jam
|
IS
|
No0001
|
Mary
|
IS
|
| No0004
|
Jam
|
IS
|
No0008
|
Katter
|
IS
|
| No0004
|
Jam
|
IS
|
No0021
|
Tom
|
IS
|
列（S1+S2）
| πSno,Sname(S1)（投影）
|
| Sno
|
Sname
|
| No0001
|
Mary
|
| No0003
|
Candy
|
| No0004
|
Jam
|
投影得到所需要的列
| σSno=No0003(S1)（选择）
|
| Sno
|
Sname
|
Sdept
|
| No0003
|
Candy
|
IS
|
选择得到所需要的行
自然连接
| 关系S1
|
| Sno
|
Sname
|
Sdept
|
| No0001
|
Mary
|
IS
|
| No0003
|
Candy
|
IS
|
| No0004
|
Jam
|
IS
|
|
| 关系S2
|
| Sno
|
Age
|
| No0001
|
23
|
| No0008
|
21
|
| No0021
|
22
|
|
| S1▷◁S2自然连接
|
| Sno
|
Sname
|
Sdept
|
Age
|
| No0001
|
Mary
|
IS
|
23
|