copyright © 2003-2008 curt hill the relational model of database basic organization and terms

Copyright © 2003-2008 Curt Hill

The Relational Model of Database

Basic organization and terms


Relational• Form is that of a table• Each table has

– A fixed number of columns• The contents of a field in a column may be

empty

– A variable number of rows

• A row is known as a tuple• A column is a set of fields and is all of

the same type• Different tables may have different

columns


Relation• In theory a relation is a set

– No ordering at all

• In practice our tuples are often ordered

• In theory a tuple is also a set• In practice a tuples are ordered

– We are able to reorder them easily


Terms• Entity• Relationship• Attribute• Tuple• Table• Relation• Often many synonyms


Definition: Schema• Schema - describes the rows of a table

– A set of attribute names• The definition of kinds of attributes

making up a tuple

– The definition of the file type– This is usually static, that is the file

definition does not change over time much

• Subschema– A piece of a larger schema

Schema Flavors• Conceptual Schema

– AKA logical schema– Describes data in terms of tables

• Physical Schema– How the data is actually stored– Viewpoint is the disk or hardware

• External Schema– A subset or different view of the

conceptual schema– Different users may see different

external schemaCopyright © 2003-2008 Curt Hill


Definition: Body• The actual tuples making up a

relation at any one time• This usually varies over time• The contents of the file can

change often and substantially


Terms Defined• Degree

– Count of attributes

• Cardinality of a relation– Number of tuples present


Keys• Candidate

– Minimal collection of one or more fields that unique identify tuple

• Primary– One of the candidate keys– Who decides?

•Business expert not db expert

• Foreign– Referenced primary in another

table


Other keys• Alternate

– A candidate key that is not the primary

• Secondary– (Not really a relational term)– Any set of attributes

• Does not have to be unique• Does not have to be a candidate key,

though candidate keys used in indices are often called secondary

– Used as a key in an index structure

copyright © 2003-2008 curt hill the relational model of database basic organization and terms

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