Database Reference
In-Depth Information
Example 12:
Give the name of all programs with course M100.
Example 13:
Give the names of department heads that are not division heads.
8.4 Substitution Rule and Standardization Rules
It is sometimes necessary to introduce “dummy” (apparently redundant) tuple variables,
in conformance with an old rule that forbids retrieval of a quantified (bounded)
variable(s). Further, for many data sub-languages (including SQL), if x and y are tuple
variables and W is a WFF involving x and y, then
This rule often simplifies the database query statement by avoiding the use of the
existential quantifier, and will become clear when we discuss SQL later in the course.
Additionally, due to the limitation of some DBMS suites in their support of relational
calculus notations, it is often useful to apply certain substitution and standardization
rules. These are mentioned below (Figure
8-2
):
