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# Special Relational Operators in Relational Database Model

The selection operator yields a 'horizontal' subset of a given relation - that is, the subset of tuples within the given relation for which a specified predicate is satisfied.

The predicate is expressed as a Boolean combination of terms, each term being a simple comparison that can be established as true or false for a given tuple by inspecting that tuple in isolation.

Book WHERE Author = ‘Kruse’

 BookID BookName Author A-112 Algorithms Jack C-12 Data Mining Jack F-348 Software Engineer Jack

Employee WHERE Desig=’Manager’ AND Dept =’SD’

 EmpNo EmpName Designation Depat 2001 Morrison Manager SD 2002 Steve Manager SD 2003 Fleming Manager SD

## Projection:

The projection yields a 'vertical' subset of a given relation- that is, the subset obtained by selecting specified attributes, in a specified left-to-right order, and then eliminating duplicate tuples within the attributes selected.

Example: Issue[BookId,ReturnDate]

 BookID ReturnDate q-110 20-May-2008 w-990 21-Jun-2008 f-100 23-Jun-2008 r-800 27-Jun-2008 q-501 15-Jul-2008

Book[BookName]

 BookName Software Concepts Data Structures Programming AssemblyLanguage SSAD PC-Troubleshooting Compiler Design

Now we know about the constructs of relational data model. We also know how to specify constraints and how to use relational algebra for illustrating various functions. We now take up another data model that is entirely different from relational model.

## Division:

The division operator divides a dividend relation A of degree m+n by a divisor relation B of degree n, and produces a result relation of degree m.

Let A be set of pairs of values <x, y> and B a set of single values, <y>. Then the result of dividing A by B - that is A DIVIDEDBY B- is the set of values x such that the pair <x, y> appears in A for all values y appearing in B.