305 Weak Entity Types PDF

** 305 Weak Entity Types ** A weak entity is an entity that cannot be uniquely identified by its own attributes alone. **...

** 305 Weak Entity Types ** A weak entity is an entity that cannot be uniquely identified by its own attributes alone. ** ** * * Unlike regular entities, weak entities depend on a strong (or owner) entity for their ** ** * == ** ** identification. == * * They do not have a primary key of their own and rely on a combination of their attributes ** ** ~~** ** ~~* == and the primary key of the owner entity to form a unique identifier. ** ** == Characteristics of Weak Entities ** Dependent on Strong Entity: A Weak Entity is associated with a Strong Entity and ** ** ** ** ** * relies on it for identification. * *** Partial Key: Weak entities have a partial key (also known as a discriminator), an * ** ** ** ** ** attribute or set of attributes that can uniquely identify the weak entity within the ** ** * context of the owner entity. * E.g. who is there *** Existence Dependency: Weak entities have an existence dependency, meaning they * ** ** ** ** ** * == cannot exist independently without their associated strong entity. ** **== * *** Double Rectangle Representation: In ER diagrams, * ** ** weak entities are represented by Double Rectangles , and ** ** == == ** their relationships with Strong Entities are shown with Double Diamonds. ** == == ** ** == == ** * Example of a Weak Entity * * * Consider a university database with two entities: Course (strong entity) and Course ` ` ** ** ` Section (weak entity). ` ** ** ** Strong Entity: Course ** ` ` ** Attributes: ** ` Course ID , ` ` Course Name ` ** Weak Entity: Course Section ** ` ` ** Attributes: ** ` Section Number (partial key), ` ** ** ` Instructor , ` ` Time ` ** In this example: ** ** ` Course Section cannot exist without a Course. ` ** * * ** ` ` ** * For example, " Section 1 " alone does not uniquely identify the section without * ` ` knowing which Course it belongs to (e.g., " Math 101 , Section 1 "). ** ` ` ** ` ` ` ` * The Course ID from Course and Section Number together uniquely identify each ** ` ` ** ** ` ` ** ** ` ` ** ** ** ** ` Course Section. ` ** * Appendix: Significance of Double Lines in ER Diagrams for Weak Entities The Double Line connecting the Weak Entity ( Course Selection ) to the relationship ( Has ) **~ ~** ` ` ` ` is a standard convention in ER (Entity-Relationship) diagrams. ** Significance of the Double Line: ** ** Dependence on Strong Entity: ** The Double Line signifies that the Weak Entity ( Course Selection ) cannot exist ** ** == ** ** ` ` ** without the corresponding strong entity ( Course ). ** ` ` == In other words, the weak entity is existence-dependent on the strong entity. ** == == ** This means every instance of the weak entity must be associated with an instance of * == the strong entity. == * ** == Primary Key Participation : * *== ** * The Double Line also indicates that the Primary Key of the Weak Entity includes the ** ** ** Primary Key of the Strong Entity. ** * This forms part of the Weak Entity’s Composite Key. ** == == ** The Strong Entity’s key acts as a Foreign Key in the Weak Entity. ** ** ** One Line for Strong Entity: ** The single line connecting the Strong Entity ( Course ) to the relationship shows that the ` ` Strong Entity can exist independently of the Weak Entity. * *