Relational algebra and calculus, Database Management System

Q. What is relational algebra and calculus? Explain the relational algebra operators :- Selection and predicate, set operation, joins and division.

Sol. Relational Algebra 

It is a procedural query language. It consists of a set of operations that tade one or two relations as input and produce a new relation as their result. The fundamental operations in the relational algebrare :- In addition to these there are several other operations namely : set intersection natural join division and assignment.                                                                                                                                                     

1. Selection and Predicate                                                                                                                                                     (i) Select operation : It selects tuples that satisfy a given predicate. We use a lower case letter sigma to denote selection . The preicate appears as subscript to entity, The argument relation is in parentheses after the query. In general, we allow comparisons using =, =, <, < _, > >, _ in the selection       predicat. We can also combine several predicate in to a larger predicate by using the connectives and or and not.                                                                                                                                                   (ii) Predicate operation :  The predicate operation is a unary operation that returns its argument relation with certain attributes left out. It is denoted by the upper cade Greek letter pi.                                                                                                                                                          

2 Set operation : The set difference operatio denoted by allows us to find tuples that are I one relation but are not in another. We can find all customers of the bank who have an account but not a loan by that branch.                                                                                                                                                                     

3 Natural join : It is a binary operatio that allows us to combine certain selections and a cartesian product into one operation. It is denoted by join symbol.                                                                                                                                                                          

4 Division operation : The division operation denoted by is suited to queries that include the pharase for all. We can obtain all branches in Brooklyn by the expression. 

Posted Date: 7/11/2012 4:06:15 AM | Location : United States

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