Database Reference
In-Depth Information
8.2 Calculus Notations and Illustrations
As in the previous chapter, the BNF notation for expressing syntactical components of a
language will be employed. In the examples that will follow to the end of the chapter, the
semicolon is used to punctuate the calculus statements in the interest of clarity. However,
bear in mind that the original QUEL language did not require such punctuations. The
salient syntactical components of relational calculus are as follows:
1.
Relational Operators
include =, <, >, <=, >=, =>, <> while
connectives
include
AND, NOT, OR
2.
Comparisons
are done variable-with-variable, attribute-with-
literal or attribute-with-attribute, separated by operators.
Note:
A comparison evaluates to true or false (similar to a
relational algebra condition). The implication operator (=>)
works as follows: P => Q means if condition P holds, then so
does condition Q. Two alternate interpretation are as follows:
•
If P is true, so is Q; otherwise P is false.
•
Either Q is true or P is false, i.e. P => Q means Q or P'.
3.
De Morgan's Law
for logical expressions is applicable:
