Relational data objects 1 Lecture 6. Relational data objects 2 Answer to last lectures activity.

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Relational data objects

Lecture 6

Answer to last lecture’s activity

Example used throughout the course

Suppliers• s_id (string); s_name (string); status (integer); city

(string)

Parts• p_id (string); p_name (string); colour (string);

weight (real); city (string)

Contracted• s_id (string); p_id (string); qty (integer)

Diagrammatic representation

Suppliers Parts

Contracted

s_name status city p_name colour

s_id p_id

Terminology

S_id S_name Status CityS1 Smith 20 LondonS2 Jones 10 ParisS3 Blake 30 ParisS4 Clark 20 LondonS5 Adams 30 Athens

Relation

Attributes

Degree

Tuples

Cardinality

Domains

Why domains?

comparisons price with price OK weight with weight OK weight with price !!!

– POSSIBLE (they are both numbers)

– BUT WRONG, i.e. MEANINGLESS

integrity of the DB

Why domains? - example

SELECT P_name, QtyFROM Parts, ContractedWHERE Parts.P_id = Contracted.P_id

SELECT P_name, QtyFROM Parts, ContractedWHERE Parts.P_id = Contracted.S_id

Suppose the P_id and S_id fields are numeric in all tables

meaningless

Domains

domains are not explicitly stored in the DB are specified as part of the DB definition (where?) each attribute - defined on a certain domain

Scalars

the smallest semantic unit of data atomic values - no internal structure with

respect to the relational model NOTE: they may have internal structure

comparison with a basic data type (Pascal) non-deterministic definition (“slippery”)

Domain = data + operators

integer: +, -, *, integer-division real char: concatenate, find string, … date: more complex data types spatial operators: all regions crossed by a line images

Domains and data types

domain (in the relational model) is the same with data type in a programming language

analyse the extent to which domain / data type definition can be supported

primitive support for domains in relational databases (INTEGER, CHAR(n), ...)

Data definition

domains are not explicitly stored in the DB are specified as part of the DB definition (where?) each attribute - defined on a certain domain

Conventions

domains unique in the DB

named relations unique in the DB

attributes unique within a relation

Domain based queries

if domains exist which relations contain information related to… a

certain domain (e.g. related to suppliers - S_id)

if domains don’t exist naming convention + attribute interrogation

Domains - advantages

domain constrained operators increased representational power

• spatial primitives for GIS; image processing features …

domain based queries

Relations

Variable vs value

variable - named object whose values can change value - element of a type

DECLARE VARIABLE example OF TYPE INTEGER// ...example := 1; // variable has certain value//...example := example * 2; // variable has certain value//...

Relation variable vs relation value

relation variable named object whose value at a given time is a relation value its values change with time

relation value

CREATE BASE RELATION example //...// insert operationsSELECT //...FROM example // the variable has a certain valueWHERE //...

Relation value

a relation value on a collection of domains (not necessarily distinct) consists of a heading and a body

Relation heading

a fixed set of attribute_name:domain_name pairs { <A1:D1>, <A2:D2>, ..., <An:Dn>}

each Ak corresponds to only one Dk

A1, ..., An are all distinct

degree (arity) - n

Relation body

a set of (n-)tuples n is the same for all the tuples in the set

n-tuple a set of attribute_name:attribute_value pairs { <A1:vi1>, <A2:vi2>, ..., <An:vin> } (tuple i)

vik must be from Dk

one and only one pair for each attribute

cardinality - no of tuples

Example - the relation Suppliers

{<s_id:STRING>, <S_name:STRING>, <Status: INT>, <City:STRING>}

heading

{ {<s_id:’s1’>, <s_name:’Smith’>, <status:20>, <city:’London’>}, {<s_id:’s2’>, <s_name:’John’>, <status:30>, <city:’Leeds’>}, {<s_id:’s3’>, <s_name:’Stella’>, <status:70>, <city:’Paris’>} }

Generate some incorrect examples

Tables as relations

relation table a table can be interpreted as a relation

there are some underlying domains each column corresponds to a certain domain the table’s heading represents the relation’s heading each row represents a tuple, etc.

Properties of relations

no duplicate tuples• SQL!!!• consequence - primary key

tuples are un-ordered attributes are un-ordered all attributes are atomic

• 1NF; why do we need 1NF?• unnormalised relation - example

Kinds of relations

named relationbase relationderived relationexpressible relationquery resultintermediate resultview (virtual)snapshot (real)stored relation

Relational database

“a database that is perceived by the user as a collection of normalised relations of assorted degrees”

(Codd, 1983)

Relational data objects 1 Lecture 6. Relational data objects 2 Answer to last lectures activity.

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