The basic concept of database design

About the design of the database in the previous study and self-study has been studied, this time through the study of the soft test in a simple review, the main simple to say a few tests used in the name.

entity, attribute, contact

Entities: Things that can be distinguished from one another objectively. Can be a specific object.

Properties: attribute of an entity, and attribute has a value domain

Contact: The relationship between entities and entities, commonly known as (1:1) (1:N) (n:m).

ER model:

Hyper-key, candidate key, primary key, foreign key , primary attribute , non-primary attribute

Hyper-Key : A property that uniquely identifies a tuple is integrated into a super-key (potentially shifting extra attributes).

candidate Key: an attribute set that uniquely identifies a tuple and does not contain extra attributes becomes a candidate key.

PRIMARY key: The user arbitrarily assigns one of several candidate keys as the primary key.

foreign key: relationship pattern R1, a property set in R2,R1 is a candidate key, but not in R2, is called a foreign key of R2

Main properties: attributes in candidate keys are called primary properties

Non-primary attribute : A property that is not contained in a candidate key in a relational schema is called a non-primary property.

Example:

Relationship mode: Student (school number, name, gender, professional number, age) Condition: Student name not repeat

Super Key (school number, name, gender)

Candidate Key (school number, name)

Primary key: School number or name, but only one.

FOREIGN Key: Professional number

Score (school number, name, course number, score)

Candidate Key: (School number, course number),(name, course number)

Primary key: School Number, course number

Main attribute: School number, name, course number

Non-primary attribute: Score

The primary key belongs to the candidate key, and the candidate key belongs to the Super key.

The basic concept of database design