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Relational data objects 1 Lecture 6

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Relational data objects 2 Answer to last lectures activity

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Relational data objects 3 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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Relational data objects 4 Diagrammatic representation

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Relational data objects 5 Terminology Relation Attributes Degree Tuples Cardinality

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Relational data objects 6 Domains

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Relational data objects 7 Why domains? comparisons price with priceOK weight with weightOK weight with price !!! –POSSIBLE (they are both numbers) –BUT WRONG, i.e. MEANINGLESS integrity of the DB

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Relational data objects 8 Why domains? - example SELECTP_name, Qty FROM Parts, Contracted WHERE Parts.P_id = Contracted.P_id SELECTP_name, Qty FROM Parts, Contracted WHERE Parts.P_id = Contracted.S_id Suppose the P_id and S_id fields are numeric in all tables meaningless

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Relational data objects 9 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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Relational data objects 10 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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Relational data objects 11 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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Relational data objects 12 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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Relational data objects 13 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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Relational data objects 14 Conventions domains unique in the DB named relations unique in the DB attributes unique within a relation

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Relational data objects 15 Domain based queries if domains exist which relations contain information related to… a certain domain (e.g. related to suppliers - S_id) if domains dont exist naming convention + attribute interrogation

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Relational data objects 16 Domains - advantages domain constrained operators increased representational power spatial primitives for GIS; image processing features … domain based queries

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Relational data objects 17 Relations

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Relational data objects 18 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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Relational data objects 19 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 operations SELECT //... FROM example// the variable has a certain value WHERE //...

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Relational data objects 20 Relation value a relation value on a collection of domains (not necessarily distinct) consists of a heading and a body

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Relational data objects 21 Relation heading a fixed set of attribute_name:domain_name pairs {,,..., } each A k corresponds to only one D k A 1,..., A n are all distinct degree (arity) - n

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Relational data objects 22 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 {,,..., } (tuple i) v ik must be from D k one and only one pair for each attribute cardinality - no of tuples

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Relational data objects 23 Example - the relation Suppliers {,,, } heading { {,,, }, {,,, }, {,,, } } body

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Relational data objects 24 Generate some incorrect examples

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Relational data objects 25 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 tables heading represents the relations heading each row represents a tuple, etc.

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Relational data objects 26 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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Relational data objects 27 Kinds of relations named relation base relation derived relation expressible relation query result intermediate result view (virtual) snapshot (real) stored relation

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Relational data objects 28 Relational database a database that is perceived by the user as a collection of normalised relations of assorted degrees (Codd, 1983)

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