In context of a relation in database, choose a false statement:

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HTET PGT Official Computer Science Paper - 2019
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  1. There can be more than one super keys.
  2. A candidate key is a minimal super key.
  3. One of the candidate keys is designated as primary key.
  4. Primary key is obtained by removing one or more attributes from a candidate key.

Answer (Detailed Solution Below)

Option 4 : Primary key is obtained by removing one or more attributes from a candidate key.
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HTET PGT Official Computer Science Paper - 2019
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Detailed Solution

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Super Key

  • It is an attribute (or set of attributes) that is used to uniquely identifies all attributes in a relation. 
  • All super keys can’t be candidate keys but its reverse is true. 
  • There can be more than one super key.
  • In relation, the number of super keys is always greater than or equal to the number of candidate keys.
  • There always exists at least one super key in a table.

Candidate key

  • It is a minimal set of attributes necessary to identify a tuple; this is also called a minimal super key.
  • Candidate key can be more than one.
  • One of the candidate keys is designated as the primary key.

Primary key

  • Candidate key from the table selected by the database administrator to uniquely identify tuples in a table known as the primary key.
  • Since the candidate is a minimal set of attributes necessary to identify a tuple therefore the primary key is also  a minimal set of attributes necessary to identify a tuple and hence primary key cannot be obtained by removing one or more attributes from a candidate key.

Therefore option 4 is false

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