Chapter 4, Problem 1RQ

Explanation of Solution

Conditions:

Following are the conditions should be met before an entity is referred as a weak entity:

• The entity should be “existence-dependent” on its parent entity.
  • If an entity is referred as an “existence-dependent”, then that entity can exist in the database if and only if when it is related with another entity existence.
• The entity should inherits a section of its primary key (PK) from its parent entity.

Example:

Consider the following ER diagram along with its attributes:

Explanation:

• The entity CLASS is “existence-dependent” on COURSE entity. Hence the CLASS entity uses a PK component from its parent entity (COURSE).
• The CLASS entity’s PK is (COURSE_ID, CLASS_SEC).
• In this strong implementation, the CLASS entity is referred as a weak entity between COURSE and CLASS.