GATE CS 2012 Q.14 (CS Q.14): Relational Algebra and Tuple Relational Calculus

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

An attribute of an entity can have more than one value

An attribute of an entity can be composite

In a row of a relational table, an attribute can have more than one value

In a row of a relational table, an attribute can have exactly one value or a NULL value

Official answer and solution

Solution

Evaluate standard properties of the Entity-Relationship (ER) model against the Relational model requirements (First Normal Form).

In the ER model, attributes of an entity can be single-valued or multivalued (e.g., an employee having multiple phone numbers). Therefore, statement (A) is valid for the ER model.
In the ER model, attributes can also be composite (e.g., an address composed of street, city, and zip code). Therefore, statement (B) is valid for the ER model.
In the relational model, relations must adhere to First Normal Form (1NF). The domain of each attribute must contain only atomic (indivisible) scalar values. Thus, a cell in a row of a relational table cannot contain a set, list, or multiset of multiple values.
Each cell must contain either exactly one atomic value or a NULL value representing an absent/unknown entry. Statement (D) is therefore a correct rule of the relational model.
Statement (C) claims that an attribute in a row of a relational table can have more than one value, which directly violates 1NF atomicity. Hence, statement (C) is INCORRECT, making it the credited answer.

Final answer

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