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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’

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]

Book[BookName]

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

• Relational Database Model
• Different Types of Keys in Relational Database Model
• Integrity Rules
• Extensions and intensions
• Key Constraints
• Relational Algebra
• Traditional set operators
• Special Relational Operators
• Object oriented Model
• Encapsulation of Operations, Methods, and Persistence
• Comparison Between Relational Database Model and Object Oriented Model

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