Unformatted text preview: Entities: If an entity can exist apart from all its related entities, then it is existence‐independent and it is referred to as strong entity or regular entity. Relationship Strengths: (Strong Relationships) The concept of relationship strength is related on how primary keys (PK) are defined among entities. To implement a relationship, the primary key of one entity ( the parent entity , normally on the “one” side of the one‐to‐many relationship) appears as a foreign key (FK)in the related entity (the “many” side on the one‐to‐many relationship). Sometimes, the foreign key is also a primary key in the related entity. Weak or Non_identifying Relationships: A weak relationship exists if the primary key of the related entity does not contain a primary key component of the parent entity. By default, relationships are established by having the primary key of the parent entity appear as a foreign key on the related entity(child entity) Example, suppose we have a 1:M relationship between COURSE and CLASS as: COURSE(CRS_CODE,DEPT_CODE,CRS_DESCRIPTION,CRS_CREDIT) CLASS(CLASS_CODE, CRS_CODE,...
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This document was uploaded on 03/07/2014.
- Spring '09