B ITM 330 Lecture Notes - Lecture 2: Compound Key, Candidate Key, Relational Model
19 Oct 2017
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Document Summary
Entity: an entity is something of importance to a user that needs to be represented in a database, an entity represents one theme or topic. In an entity-relationship model, entities are restricted to things that can be represented by a single table: a relation is a two-dimensional table that has specific characteristics, the table dimensions, like a matrix, consist of rows and columns. A key is one (or more) column(s) of a relation that is (are) used to identify a row: unique key- data value is unique for each row. Uniqueness of keys: nonunique key- data value may be shared among several rows. Consequently, the uniquely identify a row. key will identify a set of rows. A composite key is a key that contains two or more attributes. For a key to be unique, it must often become a composite key. Composite key characteristics: to identify a family member, you need to know a familyid, a firstname, and a.
