Relational_Algebra - Introduction to Relational Algebra...

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Introduction to Relational Algebra
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Overview b Introduction b Unary Relational Operations s SELECT, PROJECT, Rename b Relational Algebra Operations from Set Theory b Binary Relational Operations b Additional Relational Operations b Examples of Queries in Relational Algebra
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Relational Algebra
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Relational Algebra b The basic set of operations for the relational model is known as the relational algebra . s enable a user to specify basic retrieval requests. b The result of a retrieval is a new relation , which may have been formed from one or more relations. b The algebra operations thus produce new relations, which can be further manipulated using operations of the same algebra. b A sequence of relational algebra operations forms a relational algebra expression , whose result will also be a relation that represents the result of a database query (or retrieval request)
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Unary Relational Operations b SELECT (symbol: σ (sigma)) b PROJECT (symbol: Π (pi)) b RENAME (symbol: ρ (rho))
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Unary Relational Operations - SELECT b Let relation r have attributes X,Y,. .,Z , and let p be a truth-valued function whose parameters are, some subset of X,Y,. .,Z . Then the restriction of s r according to p s r WHERE p b – is a relation with the same heading as r and with body consisting of all tuples of r such that p evaluates to TRUE for the tuple in question. b The expression p is a predicate
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Unary Relational Operations - SELECT b SELECT operation is used to select a subset of the tuples from a relation that satisfy a selection condition . b It is a filter that keeps only those tuples that satisfy a qualifying condition – those satisfying the condition are selected while others are discarded. b The select operation is denoted by: s σ <selection condition>(R) s where the symbol σ (sigma) – denote the select operator s <selection condition> - Boolean expression specified on the attributes of relation R
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Unary Relational Operations - SELECT b The boolean expression specified in <selection condition> is made up of: s <attribute name> <comparion op> <constant value>, or s <attribute name> <comparion op> <attribute name> s where < attribute name > - any attribute of R s < comparion op > - { =, <, <=, >, >=, != } b Clauses can be connected by Boolean operators AND, OR, and NOT: b σ DNO = 4 AND SALARY>2500 (EMPLOYEE)
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Unary Relational Operations - SELECT b Example: To select the EMPLOYEE tuples whose department number is four or those whose salary is greater than $30,000 the following notation is used: b σ DNO = 4 OR SALARY > 30,000 (EMPLOYEE) s The SELECT operator is unary – applied to a single relation s The degree of the relation resulting from SELECT is the same as that of R s The number of tuples in the resulting relation is always less or equal to the number of tuples in R
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Unary Relational Operations – PROJECT b Selects certain
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Relational_Algebra - Introduction to Relational Algebra...

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