Unformatted text preview: CLASS‐SECTION, CLASS_TIME, ROOM_CODE, PROF_NUM) In this case, a weak relationship exists between COURSE and CLASS, because the PK of COURSE is a FK for CLASS. In the CROW notation, a weak relationship is represented by placing a dashed relationship line between entities COURSE PK CLASS CRS_CODE PK FK1 DEPT_CODE CRS_CREDIT CRS_CODE generates CRS_DESCRIPTION CLASS_CODE CLASS_SECTION CLASS_TIME ROOM_CODE PROF_NUM Strong (Identifying ) Relationships: A strong relationship exists when the primary key of the related entity contains a primary key component of the parent entity. For example, suppose the 1:M relationship between CLASS and COURSE is defined as: COURSE(CRS_CODE, DEPT_CODE, CRS_DESCRIPTION, CRS_CREDIT) CLASS(CRS_CODE, CLASS_CODE, CLASS‐SECTION, CLASS_TIME, ROOM_CODE, PROF_NUM) In this case, the CLASS entity primary key is composed of CRS_CODE and CLASS_SECTION. Therefore, a strong relationship exists between COURSE and CLASS. Weak Entities: In contrast to strong or regular entities mentioned earlier, a weak entity...
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This document was uploaded on 03/07/2014.
- Spring '09