Sixth ER model of database design

Entities are represented by boxes in ER diagrams

Entities are actually equivalent to a two-dimensional table, and entity instances are equivalent to one row in a two-D table

Properties are represented by ellipses in a two-dimensional table, and properties are data items that describe an entity's characteristics

Concept: Key (also become candidate key): 1, attribute set K on row only 2, no true subset of K has condition 1

Super key: Satisfies 1.

Primary key: The designer of the database chooses a candidate key that is a unique identifier for a specific row in table T

Null value: Unknown or undefined

The identifier is the primary key, and the other is the descriptor in the ER diagram

Sixth ER model of database design