The Relational Model DB Chapter 2 (and some from chapter 4, 5) J.G. Zheng June 27 th 2005

Overview Relational model terminologies and foundations Some design concepts

Introduction Edgar F. Codd, 1970 One sentence to explain relational database model: Data are organized in relations (tables), which are linked (relationship) by keys

Relation A relation is a two-dimensional table that has specific characteristics: The table consist of rows and columns Rows contain data about an entity instances All values in a row describes the same entity instance Columns contain data about attributes of the entity All values in a column are of the same kind

Relation (continued) Relations specific characteristics go on: Cells of the table hold a single value Each row is distinct Each column has a unique name The order of the rows is unimportant The order of the columns is unimportant

Non-Relation Examples

Relation Examples And, any tables in page 92 and 93

Relation Terminology Contrast TableRowColumn *EntityRecordField Relation[Tuple]Attribute

Keys A key is one or more columns of a relation that is used to identify a record Primary key Foreign key

Primary Key Primary key The value of this key column uniquely identifies a single record (row) There is only one primary key for a table Candidate Key A candidate to become the primary key There can be multiple candidate keys for a table Alternate key

Composite Key A key that contains two or more attributes (columns) Example FirstName + LastName FirstName + LastName + BirthDate FirstName + LastName + BirthDate + BirthCity …

Surrogate Key/Artificial Key Its a key created arbitrarily to replace the natural key Typically used in place of a composite key Usually it has no real meaning Example We can create a ReviewID in the BookReviews table to replace the original composite key

Relationship and Foreign Key Relationship defines how tables (relations) are linked Two tables are linked by a pair of keys The primary key of one table The foreign key in the linked table These two keys are of the same kind

Relationship Example Primary Key (PK) Foreign Key

Relationship Types One-to-one Example: students and GSU network accounts One-to-many Example: students and diplomas *Many-to-many Example: students and professors

Referential Integrity Every value of a foreign key must match a value of the primary key For example (Premiere Products database) In Customer table, RepNum is a foreign key (linked to the Rep table where RepNum is the primary key). Then every value of RepNum in the Customer table must exist in the Rep table

Relational Algebra Selection Projection Join …

Exercise Define and enforce relationship for your database assessment tables

Using MS Access Exercise: using query designer (QBE) to query AmazonBooks Choosing columns Specifying criteria Sorting Calculating

Summary Understand three important concepts of relational database model Relation Keys Primary key, candidate key, alternate key Composite key Natural key, surrogate key, artificial key Foreign key Relationship

Good Resources Edgar Frank Codd MS Access tutorial