INFS1603 Lecture Notes - Lecture 5: Subtyping, Unique Key, Social Security Number

• School: UNSW, UNSW Sydney
• Department: Information Systems
• Course: INFS1603
• Professor: All

Document Summary

The extended entity relationship model (eerm) is the result of adding more semantic constructs to the original entity relationship (er) model. An extended entity relationship diagram (eerd) is a diagram that uses the eerm. Supertype: a more generic entity type compared to its subtypes. The grouping of employees into various type provide two important benefits: It avoids unnecessary nulls in attributes when some employees have characteristics that are not shared by other employees. It enables a particular employee type to participate in relationships that are unique to that employee type. Entity supertypes and subtypes are organised in a specialisation hierarchy which depicts the arrangement of higher-level entity supertype (parent entities) and lower level entity subtypes (child entities). The relationships (cid:449)ithi(cid:374) the spe(cid:272)ialisatio(cid:374) are des(cid:272)ri(cid:271)ed i(cid:374) ter(cid:373)s of a(cid:374) (cid:858)is a(cid:859) relatio(cid:374)ships. Support attribute inheritance: define a special supertype attribute known as the subtype discriminator, define disjoint/overlapping constraints and complete/partial constraints.