Using the basic ER and relational models, which of the following statements is…

Concept

The ER model describes properties at the conceptual level: an attribute may be simple, composite, single-valued, or multivalued. A relational table instead stores tuples over domains, so each tuple-column intersection contains one atomic domain value; NULL is a marker for missing or inapplicable information, not a collection of values.

Application

• “An attribute of an entity can have more than one value” describes a multivalued ER attribute, such as several phone numbers associated with one Person entity.

• “An attribute of an entity can be composite” describes an ER attribute such as Address decomposed into Street, City, and Postal code.

• “In a row of a relational table, an attribute can have more than one value” conflicts with the basic relational-model requirement that one field contain one atomic domain value.

• “In a row of a relational table, an attribute can have exactly one value or a NULL value” accords with the basic relational representation of a field.

Cross-check

When a conceptual attribute has several values, relational mapping places those values in separate tuples, commonly in a separate relation carrying the owning entity’s key. It does not store the set inside one field. Therefore, the incorrect statement is “In a row of a relational table, an attribute can have more than one value.”