What four conditions should exist to convert a relationship to an associative entity

Chapter 4 Entity Relationship (ER) Modeling

Answers to Review Questions

1.What two conditions must be met before an entity can be classified as a weak entity? Give an

example of a weak entity.

To be classified as a weak entity, two conditions must be met:

1.The entity must be existence-dependent on its parent entity.

2.The entity must inherit at least part of its primary key from its parent entity.

For example, the (strong) relationship depicted in the text’s Figure 4.10 shows a weak CLASS

entity:

1.CLASS is clearly existence-dependent on COURSE. (You can’t have a database class unless a

database course exists.)

2. The CLASS entity’s PK is defined through the combination of CLASS_SECTION and

CRS_CODE. The CRS_CODE attribute is also the PK of COURSE.

The conditions that define a weak entity are the same as those for a strong relationship between an

entity and its parent. In short, the existence of a weak entity produces a strong relationship. And if

the entity is strong, its relationship to the other entity is weak. (Note the solid relationship line in the

text’s Figure 4.10.)

Keep in mind that whether or not an entity is weak usually depends on the database designer’s

decisions. For instance, if the database designer had decided to use a single-attribute as shown in

the text’s Figure 4.8, the CLASS entity would be strong. (The CLASS entity’s PK is

CLASS_CODE, which is not derived from the COURSE entity.) In this case, the relationship

between COURSE and CLASS is weak. (Note the dashed relationship line in the text’s Figure 4.8.)

However, regardless of how the designer classifies the relationship – weak or strong – CLASS

is always existence-dependent on COURSE.

2. What is a strong (or identifying) relationship, and how is it depicted in a Crow’s Foot ERD?

A strong relationship exists when en entity is existence-dependent on another entity and inherits at

least part of its primary key from that entity. The Visio Professional software shows the strong

relationship as a solid line. In other words, a strong relationship exists when a weak entity is related

to its parent entity. (Note the discussion in question 1.)

Under what conditions is a relationship converted to an associative entity type?

When can associative entities occur?

How are associative entities created?