You
are here: Freetutes.com
> Systems
Analysis and Design
Special Relational Operators in Relational Database Model
Selection:
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 
A112 
Algorithms 
Jack 
C12 
Data Mining 
Jack 
F348 
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 lefttoright
order, and then eliminating duplicate tuples within the attributes selected.
Example: Issue[BookId,ReturnDate]
BookID 
ReturnDate 
q110 
20May2008 
w990 
21Jun2008 
f100 
23Jun2008 
r800 
27Jun2008 
q501 
15Jul2008 
Book[BookName]
BookName 
Software Concepts 
Data Structures 
Programming 
AssemblyLanguage 
SSAD 
PCTroubleshooting 
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.
See Also
<< Previous Page
 Contents  Next
Page >>
