UH COSC 6340 - The Relational Model Chapter 3 (15 pages)

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The Relational Model Chapter 3



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The Relational Model Chapter 3

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Pages:
15
School:
University of Houston
Course:
Cosc 6340 - Database Systems

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The Relational Model Chapter 3 lational Data Model R Ramakrishnan and J Gehrke with Dr Eick s additions Why Study the Relational Model Most widely used model Vendors IBM Informix Microsoft Oracle Sybase etc Legacy systems in older models E G IBM s IMS Recent competitor object oriented model ObjectStore Versant Ontos A synthesis emerging object relational model Informix Universal Server UniSQL O2 Oracle DB2 lational Data Model R Ramakrishnan and J Gehrke with Dr Eick s additions The SQL Query Language Developed by IBM system R in the 1970s Need for a standard since it is used by many vendors Standards SQL 86 SQL 89 minor revision SQL 92 major revision current standard SQL 99 major extensions lational Data Model R Ramakrishnan and J Gehrke with Dr Eick s additions Summary Relational Data Model I 1 A relation database consists of a set of relations tables 2 Each relation consist of a set of attribute domain pairs 3 Attributes have to be single valued Additionally null values are supported 4 The attributes of a relation are unordered 5 Attribute values have to be atomic cannot be tuples or relations 6 Relations store sets of tuples queries return bags of tuples each tuple of a relation stores a value for each attribute of the relation all tuples stored in a relation must be different 7 Tuples in a relation are unordered lational Data Model R Ramakrishnan and J Gehrke with Dr Eick s additions Summary Relational Data Model II 8 Each relation has one primary key that consists of a set of attributes that uniquely identifies the tuples in a relation 9 Relationships between object in the real world are represented in the relational data model by exporting primary keys of the objects that participate in the relationship The exported keys are called foreign keys 10 Multi values attributes can be presented either by using a separate relation or by representing the object through multiple tuples one for each value of the multi valued attribute 11 Optional attributes attributes



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