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If there is more than one key for relation schema in DBMS then each key in relation schema is classified as

  1. prime key
  2. super key
  3. candidate key
  4. primary key
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4 Comments

Sir,

What I have understood from the definition of a key is that it’s a set of attributes that can uniquely identify each tuple in a relation uniquely.

 Suppose in relation R{A, B, C, D}, one candidate key is {A, B}, so it is a super key also but {A, B, C} this is a super key but not a candidate key.

Every candidate key is super key but the converse is false.

So, If a relation has 5 keys then I can guarantee that all of them are super keys but can not guarantee that all of them are candidate keys.
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Well the word “key” in the question implies “candidate key” and it is the norm followed in standard books too. “key” defaults to “candidate key”.

And 2 keys should mean 2 candidate keys. But I don’t want to answer this question as most times PSU questions boils down to guess work which will do more harm than good for exams like GATE.
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Yes sir.
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1 Answer

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CANDIDATE KEY is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes. The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key. A table can have multiple candidate keys but only a single primary key

Both candidate key and super key can be more than one in a table

super-key-and-candidate-key

A super key with no redundant attribute is known as candidate key. Candidate keys are selected from the set of super keys, the only thing we take care while selecting candidate key is that the candidate key should not have any redundant attributes.Every candidate key is a super key but, every super key may or may not be a candidate key.All candidate keys are super keys.  so option C is more correct than Option B

A primary key is selected from the set of candidate keys.only one key possible per table or we can say it can not be possible more than one for one table so option A,D are wrong.

Option C

 

2 Comments

“.Every candidate key is a super key but, every super key may or may not be a candidate key.All candidate keys are super keys

 

Instead of this Ramayan khata , we can say CK → SK and all the rest is understood from this implication 

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“If there is more than one key for relation schema in DBMS then each key in relation schema is classified as”

What’s the meaning of “classified” here?

“If there is ONLY one key for relation schema in DBMS then the key in relation schema is classified as”?

When is a key in a relation schema classified as “super key”?

 

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Answer:

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