数据库各种码与关系代数
作者：互联网
Key
- K is a superkey of R if values for K are sufficient to identify a unique tuple
- K is a candidate key if superkey K is minimal
- Primary Key is the candidate key chosen by the database designer. We should choose the attribute set whose value rarely change
Foreign Key Constraint
- Foreign key constraint
The value of a foreign key must be a value of the primary key of the referenced relation, or be null - A relation can has zero, one, or more foreign keys
advisor (s_id, i_id) - A relation can reference itself.
employee (ID, name, …, manager)
manger is the ID of the employee’s manager
Referential integrity constraint
实体完整性：实体完整性是指关系的主关键字不能重复也不能取“空值"。
参照完整性：参照完整性是定义建立关系之间联系的主关键字与外部关键字引用的约束条件。
Relational Algebra（关系代数）
Select Operation
Project Operation
- The result is defined as the relation of k columns obtained by erasing the columns that are not listed
- Duplicate rows removed from result, since relations are sets
Generalized Projection
Composition of Relational Operations
Cartesian-Product Operation
Natural Join Operation
Outer Join
- Left Outer Join
- Right Outer Join
- Full Outer Join
Assignment Operation
Aggregate Operation
标签：关系,Join,完整性,数据库,relation,key,Outer,Operation,代数 来源： https://blog.csdn.net/weixin_46054349/article/details/122105453