Database Reference
In-Depth Information
Example 8:
Suppose that we need to find all students who share birthdays with at
least one other student. The solution is obtained by taking the cross product of
Student
and itself, and then searching for matches on
DoB
:
7.4.2 The Assignment Operation
The assignment operator is a colon followed by an equal sign (:=), and is used to have the
system remember the results of other operation(s), which may be required for later use.
The format of the assignment is:
The example in the upcoming subsection illustrates how relational assignments are
done.
7.4.3 The Rename Operation
The
RENAME
operation renames specified attributes of a specified relation R1, into a
new relation R2. We have established that it is desirable to have unique attribute names
in each relation. Further, in the case of nested operations, it is a good habit to have an
unambiguous way of referring to attributes of the results of an inner expression, from an
outer expression. The
RENAME
operation is ideal for these situations. The format of the
RENAME
operation (based on Date's notation) is:
