Database Reference
In-Depth Information
4.
Boolean Expressions
can be derived by combination of
comparisons using connectors AND, OR, NOT:
Note:
Boolean expressions may be bracketed in the interest of
clarity. This is necessary in situations where there are nested
expressions.
5.
Range Definition
is as follows:
6.
Relational Expression:
A tuple calculus relational expression
is of the following form:
Note:
•
Where the tuple variable alone is specified, all attributes
associated with the variable are implied (remember that a
tuple variable is defined on a relation which has attributes).
•
The relational expression defines a projection on attributes
in the target list from the Cartesian product of all referenced
relations (from the target list). The WFF (
well formed
formula
) dictates a selection from the Cartesian product.
7.
Well-Formed Formula
(WFF): A well-formed formula (WFF)
is of the following form:
