GATE2012-14

Question

Given the basic ER and relational models, which of the following is INCORRECT?

• A. An attribute of an entity can have more than one value
• B. An attribute of an entity can be composite
• C. In a row of a relational table, an attribute can have more than one value
• D. In a row of a relational table, an attribute can have exactly one value or a NULL value

Comments

This might help ....

Answer 1

(C) is incorrect as a relational table requires that, in a row, an attribute can have exactly one value or NULL value.

Comments

 y it is incorrect "In a row of a relational table, an attribute can have more than one value" then wts mean of multivalued attribute y it cnt state multivalued atribute?????????

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Multi-valued attribute is not possible in a relational table which means any relation in relational model is by default in 1NF.

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then when multivalued attribute or wts use y we use such term please say bcz i search but didnt get satisfactory answer

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Say you have two phone numbers {981456234 and 345234981}. So, now phone number filed becomes multi-attribute. But in a relational model, this can't be stored in a single row. It will be stored as

Shimpy 981456234
Shimpy 345234981

But there can be other models like ER model where data can be represented as

Shimpy -  {981456234 and 345234981} which is not in 1NF.

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so it imeans it is necessary tht ER model must be in 1 NF always??????????????????? & to make this condition we are making c incorrect???

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No, a relation in ER model needn't always be in 1NF, but a relation in relational model always will be. And this makes C INCORRECT.

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thank you :)

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The formalized definition of a relation guarantee that the attribute values should be atomic (non-divisible). So, multi-valued are not allowed for same tuple. However, an attribute can have multiple values for different tuples.

Answer 2

The term ‘entity’ belongs to ER model and the term ‘relational table’ belongs to relational model.

A and B both are true. ER model supports both multivalued and composite attributes See this for more details.

(C) is false and (D) is true. In Relation model, an entry in relational table can can have exactly one value or a NULL.

Comments

You say above that "see this for more details" ? But no link there. Mention plz if any.

Answer 3

Option C is correct.

According to Relational Model every relation should be in 1NF and according to 1 NF, there should not be a multivalued attribute in a relation. That means an attribute can have only one value or NULL value in a row. So statement of option C is incorrect

Answer 4

Option C is correct.

 

 First Two options are related to ER Model and In E-R diagram we can have multi valued and composite attributes so those two options are correct.

            Last Two options are related to Relational model 

According to Relational Model every relation should be in 1NF and according to 1 NF, there should not be a multi-valued attribute in a relation. That means an attribute can have only one value or NULL value in a row. So statement of option C is incorrect