Relational data objects

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Relational data objects

Lecture 6

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Answer to last lecture’s activity

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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)

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Diagrammatic representation

Suppliers Parts

Contracted

s_name status city p_name colour

s_id p_id

qty

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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

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Domains

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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

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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

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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

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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”)

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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

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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), ...)

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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

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Conventions

domains unique in the DB

named relations unique in the DB

attributes unique within a relation

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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

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Domains - advantages

domain constrained operators increased representational power

• spatial primitives for GIS; image processing features …

domain based queries

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Relations

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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//...

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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 //...

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Relation value

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

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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

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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

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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’>} }

body

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Generate some incorrect examples

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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.

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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

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Kinds of relations

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

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Relational database

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

(Codd, 1983)