Database Reference
In-Depth Information
As you will later see (in chapter 14), it is application of this methodology that assists
in the implementation of sophisticated system catalogs that characterize contemporary
DBMS suites. However, with this knowledge, you can actually model and design
databases to mirror E-relations and P-relations as described. One obvious advantage
is that if you used one P-relation instead of one for each E-relation, then in accessing
the database for actual data, you would be accessing fewer relations (in fact just one
relation) than if you had used another approach (such as the E-R model). The flip side to
this advantage is that this relation would be extremely large for a medium sized or large
database; this could potentially offset at least some of the efficiency gained from just
having to access one relation for data values.
5.4.4 Integrity Rules
With this set-up, accessing and manipulating data in the database is accomplished by the
DBMS through the E and P relations. For this reason, additional integrity rules must be
imposed. The complete list of integrity rules follows:
1.
Entity integrity rule (review section 4.1)
2.
Referential rule (review section 4.1)
3.
XR Model Entity Integrity: E-relations accept insertions and
deletions but no updates (surrogates don't change)
4.
Property Integrity: A property cannot exist in the database
unless the tuple (entity) it describes exists
5.
Characteristic Integrity: A characteristic entity (tuple) cannot
exist unless the entity (tuple) it describes exists
6.
Association Integrity: An association entity (tuple) cannot
exist unless each participating entity (tuple) also exists
7.
Designation Integrity: A designative entity (tuple) cannot exist
unless the entity (tuple) it designates also exists
8.
Subtype Integrity: A subtype entity (tuple) cannot exist except
there be a corresponding super-type entity (tuple)
The following rules apply to subtypes and super-types:
1.
All characteristics of a super-type are automatically
characteristics of the corresponding subtype(s), but the
converse does not hold.
2.
All associations in which a super-type participates are
automatically associations in which the corresponding
subtype(s) participate, but the converse does not hold.
