ER Model and Business Rules Questions and Answers

Answer: D

The correct ER diagram is (iv).

The specification has four entity sets, Instructor, Course, Classroom and Student, and four relationships: Teaches (Instructor, Course), Registration (Student, Course), Allocation (Course, Classroom) and Guides (Instructor, Student). Diagram (iv) draws exactly these, with entities as rectangles and relationships as diamonds joining the right pairs.

Diagram (i) makes Registration an entity and Student a relationship. Diagram (ii) has no Student entity and has Guides linking Instructor to Projects. Diagram (iii) leaves Allocation unconnected to Classroom and joins Classroom directly to Course.

Answer: A

The true statement is The weak entity set MUST have total participation in the identifying relationship.

A weak entity has no key of its own; it is identified only through its owner, by combining the owner's key with its partial key. So every weak entity must be linked to an owner through the identifying relationship, which is total participation.

The owner need not participate totally: an employee may have no dependents, for example, so not every owner entity has a related weak entity. That rules out the options requiring total participation of the owner, and 'neither' is wrong because the weak side's participation is required.

Answer: A

The true statement is Every entity in E1 is associated with exactly one entity in E2.

Many-to-one from E1 to E2 means each E1 entity is related to at most one E2 entity. Total participation of E1 means each E1 entity is related to at least one. Together, each E1 entity has exactly one partner in E2.

Some entity in E1 with more than one E2 partner would break the many-to-one rule. On the E2 side, E1 has more entities than E2 and every E1 entity is mapped to some E2 entity, so by the pigeonhole principle some E2 entity is related to two or more E1 entities. That rules out 'exactly one' and 'at most one' for E2.

Answer: C

Correct answer: Entity

The rectangle is the entity symbol in an entity-relationship diagram, as the oval is the attribute symbol and the diamond the relationship symbol. Optional participation is shown by cardinality notation on the relationship line rather than by any shape of its own.

Answer: A

Correct answer: Attribute

The oval is the attribute symbol, attached by a line to the entity or relationship it describes. The three basic shapes are quickly learnt: rectangle for entity, oval for attribute, diamond for relationship - and the variants such as the double oval for a multivalued attribute build on them.

Answer: B

Correct answer: False

Clustering groups related entities into a single higher-level object, so the diagram shows fewer symbols. The point is to make a large model readable at a summary level, with the detail visible on decomposing a cluster rather than all at once.

Answer: A

Correct answer: True

Historically the rules were coded into each application that touched the data, which is why the same rule was often implemented several times and inconsistently. Moving them into the database as constraints and triggers, or into a rules engine, is what gives one authoritative statement of each rule.

Answer: A

Correct answer: True

An entity cluster is a set of entity types and their relationships treated as one object on a summary diagram. Because a cluster can itself contain clusters, a large model can be presented at several levels of detail, each readable on its own.

Answer: B

Correct answer: Attributes are assigned at the highest logical level.

An attribute shared by every subtype belongs at the supertype, where it is held once and inherited by all. Pushing it down to the subtypes duplicates it and invites inconsistency. The rule is a matter of placing each attribute as high as it is true, and no higher.

Answer: A

Correct answer: True

This is one of the two conditions justifying the construct. If an attribute applies to only some instances, holding it on the single entity type means most rows carry a null, and the null has no meaning - it is not unknown but inapplicable. A subtype gives it a proper home.