Which of the following options are correct regarding these three keys (Primary Key, Super Key, and Candidate Key) in a database?I. Minimal super key is a candidate keyII. Only one candidate key can be a primary keyIII. All super keys can be a candidate keyIV. We cannot find a primary key from the candidate keyI and IIII and IIII and IIIII and IV
Question
Which of the following options are correct regarding these three keys (Primary Key, Super Key, and Candidate Key) in a database?I. Minimal super key is a candidate keyII. Only one candidate key can be a primary keyIII. All super keys can be a candidate keyIV. We cannot find a primary key from the candidate keyI and IIII and IIII and IIIII and IV
Solution
The correct options regarding these three keys (Primary Key, Super Key, and Candidate Key) in a database are:
I. Minimal super key is a candidate key This is correct. A minimal super key is a set of attributes (i.e., columns in a table) that can uniquely identify a tuple (i.e., a row in a table) with minimal attributes. This is the definition of a candidate key.
II. Only one candidate key can be a primary key This is also correct. While a table can have multiple candidate keys, only one of them can be chosen as the primary key.
III. All super keys can be a candidate key This is incorrect. While a candidate key is a type of super key, not all super keys are candidate keys. A super key is a set of one or more columns that can uniquely identify a record. A candidate key is a minimal set of columns that can uniquely identify a record. Therefore, if a super key contains additional columns that are not necessary for unique identification, it is not a candidate key.
IV. We cannot find a primary key from the candidate key This is incorrect. The primary key is chosen from the set of candidate keys. Therefore, we can find a primary key from the candidate keys.
So, the correct answer is "I and II".
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