Database Reference
In-Depth Information
Observe that both the Ullman notation and the Date notation convey the point that
a join is really a cross product followed by a selection; the Russell notation implies it.
In each of the three cases, the result is a set of tuples from the cross product of the first
relation with the second relation that satisfy the given condition. Moreover, with this
definition, we can bring further clarity to the equijoin and the natural join as follows:
•
The equijoin is simply the specific case where
theta
(the
operator) is the equal operation. It is applicable in situations
where matching attributes do not have identical names; typically,
one attribute (or combination of attributes) constitutes a foreign
key in one relation, while the other attribute (or combination of
attributes) constitutes a candidate key in the other relation.
•
The natural join is a special case of the equijoin, where the
attributes compared from both relations are not just defined on
the same domain, but have the same name. Further, the natural
join will eliminate one of the two (sets of ) identical attributes
from the final result.
Example 5b:
Let us revisit the problem in example 5a — constructing a list showing
combinations of department name and name of the related department head. This
second solution involves an equijoin.
7.3.6 Union, Intersection, Difference Statements
The
UNION, INTERSECT and MINUS
statements are similarly represented in Ullman's
notation as well as Date's notation. The respective syntax forms are as follows:
