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# ch3-2 - Chapter 3 Relational Model Structure of Relational...

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©Silberschatz, Korth and Sudarshan 3.1 Database System Concepts Chapter 3: Relational Model Chapter 3: Relational Model Structure of Relational Databases Relational Algebra Tuple Relational Calculus Domain Relational Calculus Extended Relational-Algebra-Operations Modification of the Database Views ©Silberschatz, Korth and Sudarshan 3.2 Database System Concepts Example of a Relation Example of a Relation

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©Silberschatz, Korth and Sudarshan 3.3 Database System Concepts Basic Structure Basic Structure Formally, given sets D 1 , D 2 , …. D n a relation r is a subset of D 1 x D 2 x … x D n Thus a relation is a set of n-tuples ( a 1 , a 2 , …, a n ) where each a i D i Example: if customer-name = {Jones, Smith, Curry, Lindsay} customer-street = {Main, North, Park} customer-city = {Harrison, Rye, Pittsfield} Then r = { (Jones, Main, Harrison), (Smith, North, Rye), (Curry, North, Rye), (Lindsay, Park, Pittsfield)} is a relation over customer-name x customer-street x customer-city ©Silberschatz, Korth and Sudarshan 3.4 Database System Concepts Attribute Types Attribute Types Each attribute of a relation has a name The set of allowed values for each attribute is called the domain of the attribute Attribute values are (normally) required to be atomic , that is, indivisible + E.g. multivalued attribute values are not atomic + E.g. composite attribute values are not atomic The special value null is a member of every domain The null value causes complications in the definition of many operations + we shall ignore the effect of null values in our main presentation and consider their effect later
©Silberschatz, Korth and Sudarshan 3.5 Database System Concepts Relation Schema Relation Schema A 1 , A 2 , …, A n are attributes R = ( A 1 , A 2 , …, A n ) is a relation schema E.g. Customer-schema = ( customer-name, customer-street, customer-city ) r ( R ) is a relation on the relation schema R E.g. customer (Customer-schema) ©Silberschatz, Korth and Sudarshan 3.6 Database System Concepts Relation Instance Relation Instance The current values ( relation instance ) of a relation are specified by a table An element t of r is a tuple , represented by a row in a table Jones Smith Curry Lindsay customer-name Main North North Park customer-street Harrison Rye Rye Pittsfield customer-city customer attributes (or columns) tuples (or rows)

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©Silberschatz, Korth and Sudarshan 3.7 Database System Concepts Relations are Unordered Relations are Unordered Order of tuples is irrelevant (tuples may be stored in an arbitrary order) E.g. account relation with unordered tuples ©Silberschatz, Korth and Sudarshan 3.8 Database System Concepts Database Database A database consists of multiple relations Information about an enterprise is broken up into parts, with each relation storing one part of the information E.g.: account : stores information about accounts depositor : stores information about which customer owns which account customer : stores information about customers Storing all information as a single relation such as bank ( account-number, balance, customer-name , ..) results in + repetition of information (e.g. two customers own an account)
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ch3-2 - Chapter 3 Relational Model Structure of Relational...

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