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: B

ER modelling represents a top down approach.

An entity-relationship design starts from the whole problem: the designer first identifies the main entities and the relationships between them, then refines them by adding attributes, keys and cardinalities, and finally maps the model to tables. Moving from the general picture to the details is the top-down method.

Normalisation is the usual example of the bottom-up approach, starting from individual attributes and grouping them into relations. 'Left right' and 'bottom left' are not design approaches.

Answer: A

The primary key is train no. + date.

A primary key must identify each reservation record uniquely. A given train number runs every day (or on several days), so the train number alone repeats; the combination of train number and date picks out one particular journey, on which seat availability, seat type and the rest depend.

Train number plus destination, or plus place of departure, still repeats from day to day, since the same train serves the same stations daily, and the train number alone is not unique either.

Answer: B

Statements (i) and (ii) are true, so the answer is (i) and (ii).

A weak entity set is existence dependent on its identifying (owner) entity set: a dependent cannot exist without the employee it belongs to (i). It has no primary key of its own (ii): it is identified by its discriminator (partial key) combined with the primary key of the owner.

Statement (iii) is false. A weak entity has its own attributes, such as a dependent's name and age; only its identification borrows the owner's key, so its attributes are not derived from the strong entity's attributes.

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.