[Database] Four Paradigms

prerequisite knowledge

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• What is a keycode?
  If an attribute or a set of multiple attributes {A,B,C,D….} satisfies the following conditions, it is called the key code of the relation R.
• These property functions determine all other properties of the relationship.

• No proper subset of A,B,C,D….} can functionally determine other properties of R, that is, {A,B,C,D….} must be minimal.

• What is the main attribute?
  The property where the key code is located becomes the primary property.

• What is a non-primary attribute?
  Properties that are not key codes are called non-primary properties.

  Example:
  
  For example, in the above figure, Snosum Cnameis the main attribute, and the others are non-main attributes. {Sno，Cname}for the key code.
  
  
  
  
  

four paradigms

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• first normal form

If a relational schema is in 所有属性都是不可分割的基本属性the first normal form

• second normal form

If a relational schema is in first normal form, where 所有非主属性都完全函数依赖于键码, is in second normal form

• third normal form

If a relational schema is in first normal form, where 所有非主属性都不传递依赖于键码, is in third normal form

• BC paradigm

If a relational schema is in first normal form, where 所有属性都不传递依赖于键码, is in BC normal form