4.20- Domain Relational Calculus In dbms In Hindi

4.20- Domain Relational Calculus In dbms In Hindi

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Domain Relational Calculus
Every query in Domain relational calculus is expressed by a domain calculus expression,
which is the basic construct. A domain calculus expression is of the form
[ ] D|F(D) or ( , , ..., )|F ( , , ..., ) { dd d dd d 1 2 n n ( 1 2 )}
Here, D is a set of domain variables (d1, d2, ..., dn) and F is a formula involving
D(d1, d2, ..., dn). The result of this query is the set of all tuples for which the formula is true.
The operators used by the Domain relational calculus are same as those used by tuple
relational calculus. A tuple relational calculus expression can be converted to a domain
calculus expression by replacing each tuple variable by n domain variables, where n is the
arity of the tuple variable.
176 Introduction to Database Management System
5.3.2.1 Basic Concepts
The definitions of Free variables, Bound variables and Qualified variables are same as
in tuple relational calculus. Here, we discuss about other definitions that are different from
tuple relational calculus.
Atom or Atomic formula : An atom or atomic formula may be in any of the following
forms:
(i) D ∈ R, where D(d1, d2, ..., dn) is the set of domain variables and R is a relation
with n attributes.
(ii) X ⊗ Y, where ⊗ is any one of comparison operators and X and
Y are domain variables.
(iii) X ⊗ Constant or Constant ⊗ X, where constant must be domain compatible.
Well formed formula (WFF) : A well formed formula (WFF) is recursively defined to
be one of the following:
(i) Every atom is a WFF.
(ii) If f is a WFF, so are (f) and ¬f.
(iii) If f and g are WFF’s, then so are f ∨ g, f ∧ g, f → g.
(iv) If f(x) is a WFF and X is free and a domain variable, then ∃X(f(x)) and ∀X(f(x)) are also WFF’s.
The major difference between tuple relational calculus and domain relational calculus is that in domain calculus the domain variables are used to represent component of tuples instead of tuples variables used in tuple calculus.
Again the relation Employee and Department shown in Figure 5.19 are used for examples on domain relational calculus.

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