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Super Key | Candidate Key | Primary Key | Composite Key

  • Super Key: A Super Key is a set of one or more attributes that can identify all other attributes uniquely within a relation. Super key stands for superset of Candidate key.
  • Candidate key: Candidate keys are defined as the set of fields from which primary key can be selected. It is an attribute or set of attribute that can act as a primary key for a table to uniquely identify each record in that table.  
  •  Primary key: Primary key is a candidate key that is most appropriate to become main key of the table. It is a key that uniquely identify each record in a table. 
PK
Primary Key

  • Composite Key:  Composite key that consist of two or more attributes that uniquely identify an entity occurance is called Composite key.
    CK
    Composite key
     

Example: Consider a Relation or Table R1. Let A,B,C,D,E are the attributes of this relation.

R(A,B,C,D,E)
A→BCDE This means the attribute 'A' uniquely determines the other attributes B,C,D,E.
BC→ADE This means the attributes 'BC' jointly determines all the other attributes A,D,E in the relation.

Primary Key :A
Candidate Keys :A, BC
Super Keys : A,BC,ABC,AD

ABC,AD are not Candidate Keys since both are not minimal super keys.
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