COP 4710 Lecture Notes - Lecture 4: Blackwall Tunnel, Candidate Key, Unique Key
Document Summary
Database = collection of entities & relationship among them. Entity= object existing and is distinguishable from other objects. An entity set is a set of entities of the same type that share the same properties. A relationship is an association among several entities. A relationship set is a mathematical relation among n>=2 entities each taken from entity sets : {(e1,e2 )| e1 e e1, e2 e e2, en e en} An attribute can also be property of a relationship set. For instance, the depositor relationship set between entity sets customer and account may have the attribute access-date. Refers to number of entity sets that participate in a relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. Relationship sets may involve more than two entity sets. Example: suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches.