The ER diagram in Figure 17.13 shows only entities and primary key attributes. The absence of recognisable named entities or relationships is to emphasize the rule-based nature of the mapping rules described in Step 2.1 of logical database design.

Question

Answer the following questions with reference to how the ER model in Figure 17.13 maps to relational tables.

(a) How many relations will represent the ER model?

(b) How many foreign primary keys are mapped to the relation representing X?

(c) Which relation(s) will have no foreign key?

(d) Using only the letter identifier for each entity, provide appropriate names for the

relations mapped from the ER model.

(e) If the cardinality for each relationship is changed to one-to-one with total participation

for all entities; how many relations would be derived from this version of the ER model?

Answer 1

(a) How many relations will represent the ER model? 6
(b) How many foreign primary keys are mapped to the relation representing X?
2 - The primary keys of entities X and D as they have a 0ne-to-many relationship with X and
they are on the one side and therefore send a copy of their primary keys to X. Once mapped to
X – the copies of primary keys are referred to as foreign keys.
(c) Which relation(s) will have no foreign key?
Relations A, C and D
(d) Using only the letter identifier for each entity, provide appropriate names for the
relations mapped from the ER model.
Relation A
Relation B
Relation C
Relation CX (Representing the many-to-many relationship between entities C and X)
Relation XE (Entity X and E merge as there is a one-to-one relationship with total (i.e. full)
participation relating these entities).
Relation D
(e) If the cardinality for each relationship is changed to one-to-one with total participation
for all entities; how many relations would be derived from this version of the ER model?

1 (Relation ABCXDE)