CS 3430 Lecture Notes - Lecture 9: Cardinality

• School: App State
• Department: Computer Science
• Course: CS 3430
• Professor: Mohammad Mohebbi

Document Summary

Degree of a relationship type is the number of participating entity types. Bi(cid:374)ar(cid:455) relatio(cid:374)ships, relati(cid:374)g two e(cid:374)tit(cid:455) t(cid:455)pes, su(cid:272)h as (cid:862)ma(cid:374)ages(cid:863) or (cid:862)works_o(cid:374)(cid:863). Relationship of degree 3, su(cid:272)h as (cid:862)a upplier supplies a part for a project(cid:863). Give an example of an instance of. Relationship types could carry some constraints from the mini-world; For e(cid:454)a(cid:373)ple, i(cid:374) the proje(cid:272)t des(cid:272)riptio(cid:374) (cid:272)ould (cid:271)e a se(cid:374)te(cid:374)(cid:272)e like this (cid:862)each employee can. Such a constraint could be represented in er with relationship cardinality ratio. It specifies the maximum number of participation of an entity instance in a relationship set: relationship cardinality ratio in er (cid:862)cardinality ratio 1: n for e1 : e2 in r(cid:863). What this means: an e1 instance could be related to n e2 instances, an e2 instance could be related to 1 e1 instance. (cid:862)an employee works_for a department(cid:863) is of (cid:272)ardi(cid:374)alit(cid:455) ratio of : Database systems 3430 chapter 3: existence dependency constraint.