Chapter 6-47
Copyright © 2004 Ramez Elmasri and Shamkant Navathe
Elmasri/Navathe,
Fundamentals of Database Systems, Fourth Edition
The Domain Relational Calculus
lAnother variation of relational calculus
called the domain relational calculus, or simply, domain calculus is equivalent to tuple
calculus and to relational algebra.
l
lThe language called QBE
(Query-By-Example) that is related to domain calculus was developed almost
concurrently to SQL at IBM Research, Yorktown Heights, New York. Domain calculus was
thought of as a way to explain what QBE does.
lDomain calculus differs from tuple calculus
in the type
of variables used in formulas: rather than having variables range over
tuples, the variables range over single values from domains of
attributes. To form a relation of degree n for a query result, we must have n of these domain variables—one for each
attribute.
l An
expression of the domain calculus is of the form
l {x1, x2, . .
., xn | COND(x1, x2, . . ., xn, xn+1, xn+2, . . ., xn+m)}
where x1, x2, . . ., xn, xn+1, xn+2, . . ., xn+m are
domain variables that range over domains (of attributes) and COND is a condition or formula of the domain relational calculus.
l
