1. Super Key (SK): 

  • Any permutation combination of attributes present in a table that can uniquely identify each tuple.


2. Candidate Key (CK):

  • The minimum subset of super keys, which can uniquely identify each tuple. It contains no redundant attribute. 
  • CK value shouldn’t be NULL.


3. Primary Key (PK): 

  • Selected out of CK set, has the least no. of attributes.


4. Alternate Key (AK)

  •  All CK except PK.


 5. Foreign Key (FK)

  • The FOREIGN KEY the constraint is used to prevent actions that would destroy links between tables.
  • A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.
  • The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.


6. Composite Key:

  •  PK is formed using at least 2 attributes. 


7. Compound Key: 

  • PK which is formed using 2 FK. 


8. Surrogate Key: 

  • 1. Synthetic PK. 
  • 2. Generated automatically by DB, usually an integer value. 
  • 3. May be used as PK.