DBDI / Rel Calculus 30/05/2007
Lecture 7 1
DBDI/ Lecture 7
Relational DB Languages
Relational Calculus & Other DB
Languages
Dr. Ala Al-Zobaidie
The slides are based on the textbook Database Systems by Connolly & Begg
30/05/2007 DBDI / Rel Calculus 2
Lecture’s Objectives
• How to form queries in tuple relational
calculus.
• How to form queries in domain relational
calculus.
• Other DB Languages
30/05/2007 DBDI / Rel Calculus 3
Relational Calculus
• If predicate contains a variable (e.g. ‘xis
a member of staff’), there must be a
range for x.
• When we substitute some values of this
range for x, proposition may be true; for
other values, it may be false.
• When applied to databases, relational
calculus has forms: tuple and domain.
30/05/2007 DBDI / Rel Calculus 4
Tuple Relational Calculus
• Interested in finding tuples for which a predicate
is true. Based on use of tuple variables.
• Tuple variable is a variable that ‘ranges over’ a
named relation: ie., variable whose only permitted
values are tuples of the relation.
• Specify range of a tuple variable Sas the Staff
relation as:
Staff(S) Range of S is the Staff table
• To find set of all tuples S such that P(S) is true:
{S | P(S)}
30/05/2007 DBDI / Rel Calculus 5
Tuple Relational Calculus - Example
• To find details of all staff earning more
than £10,000:
{S | Staff(S) ∧S.salary > 10000}
• To find a particular attribute, such as
salary, write:
{S.salary | Staff(S) ∧S.salary > 10000}
If true produce S
30/05/2007 DBDI / Rel Calculus 6
Tuple Relational Calculus
• Can use two quantifiers to tell how many
instances the predicate applies to:
– Existential quantifier ∃(‘there exists’)
– Universal quantifier ∀(‘for all’)
• Tuple variables qualified by ∀or ∃ are
called bound variables, otherwise called free
variables.
