1. Inclusion Dependencies
2. Template Dependencies
3. Functional Dependencies Based on Arithmetic Functions and Procedures
4. Domain-Key Normal Form

**Other Dependencies and Normal Forms**

We already introduced another type of dependency called join dependency
(JD) in Section 15.7. It arises when a relation is decomposable into a set of
projected relations that can be joined back to yield the original relation.
After defining JD, we defined the fifth normal form based on it in Section
15.7. In the present section we will introduce some other types of dependencies
that have been identified.

**1. Inclusion
Dependencies**

Inclusion
dependencies were defined in order to formalize two types of interrelational
constraints:

The foreign key (or referential integrity)
constraint cannot be specified as a functional or multivalued dependency
because it relates attributes across relations.

The constraint between two relations that represent
a class/subclass relation-ship (see Chapters 8 and 9) also has no formal
definition in terms of the functional, multivalued, and join dependencies.

**Definition. **An** inclusion dependency ***R*.*X***
**<** ***S*.*Y*** **between two sets of attributes—*X* of relation schema *R*,
and *Y* of relation schema *S—*specifies the constraint that, at any
specific time when *r* is a relation
state of *R* and *s* a relation state of *S*,
we must have

π* _{X}*(

The ⊆ (subset) relationship does not necessarily have to
be a proper subset. Obviously, the sets of attributes on which the inclusion
dependency is specified—*X* of *R* and *Y* of *S*—must have the same
number of attributes. In addition, the domains for each pair of corresponding
attributes should be compatible. For exam-ple, if *X* = {*A*_{1}, *A*_{2}, ..., *A _{n}*} and

For
example, we can specify the following inclusion dependencies on the relational
schema in Figure 15.1:

DEPARTMENT.Dmgr_ssn < EMPLOYEE.Ssn

WORKS_ON.Ssn < EMPLOYEE.Ssn

EMPLOYEE.Dnumber < DEPARTMENT.Dnumber

PROJECT.Dnum < DEPARTMENT.Dnumber

WORKS_ON.Pnumber < PROJECT.Pnumber

DEPT_LOCATIONS.Dnumber < DEPARTMENT.Dnumber

All the
preceding inclusion dependencies represent **referential
integrity** **constraints**. We can
also use inclusion dependencies to represent** class/subclass relationships**. For example, in the relational
schema of Figure 9.6, we can specify** **the
following inclusion dependencies:

EMPLOYEE.Ssn < PERSON.Ssn

ALUMNUS.Ssn < PERSON.Ssn

STUDENT.Ssn < PERSON.Ssn

As with
other types of dependencies, there are *inclusion
dependency inference rules* (IDIRs). The following are three examples:

IDIR1 (reflexivity): *R*.*X*
< *R*.*X*.

IDIR2 (attribute correspondence): If *R*.*X*
< *S*.*Y*, where *X* = {*A*_{1}, *A*_{2}, ..., *A _{n}*} and

*Y *= {*B*_{1},* B*_{2},
...,* B _{n}*} and

IDIR3 (transitivity): If *R*.*X*
< *S*.*Y*
and *S*.*Y*
< *T*.*Z*, then *R*.*X*
< *T*.*Z*.

The
preceding inference rules were shown to be sound and complete for inclusion
dependencies. So far, no normal forms have been developed based on inclusion
dependencies.

**2. Template
Dependencies**

Template
dependencies provide a technique for representing constraints in relations that
typically have no easy and formal definitions. No matter how many types of
dependencies we develop, some peculiar constraint may come up based on the
semantics of attributes within relations that cannot be represented by any of
them. The idea behind template dependencies is to specify a template—or
example—that defines each constraint or dependency.

There are
two types of templates: tuple-generating templates and constraint-generating
templates. A template consists of a number of **hypothesis tuples** that are meant to show an example of the tuples
that may appear in one or more relations. The other part of the template is the
**template conclusion**. For
tuple-generating templates, the conclusion is a *set of tuples* that must also exist in the relations if the
hypothesis tuples are there. For constraint-generating templates, the template
conclusion is a *condition* that must
hold on the hypothesis tuples. Using constraint-generating templates, we are
able to define **semantic constraints**—those
that are beyond the scope of the relational model in terms of its data
definition language and notation.

Figure
16.5 shows how we may define functional, multivalued, and inclusion
dependencies by templates. Figure 16.6 shows how we may specify the constraint
that an *employee’s salary cannot be
higher than the salary of his or her direct supervisor* on the relation
schema EMPLOYEE in Figure 3.5.

**Figure 16.5**

Templates for some common
type of dependencies.

Template for functional
dependency X → Y.

Template for the
multivalued dependency X →→
Y.

Template for the inclusion
dependency R.X < S.Y.

**Figure 16.6**

Templates for the
constraint that an employee’s salary must be less than the supervisor’s salary.

**3. Functional Dependencies
Based on Arithmetic Functions and Procedures**

Sometimes
some attributes in a relation may be related via some arithmetic function or a
more complicated functional relationship. As long as a unique value of *Y* is associated with every *X*, we can still consider that the FD *X* → *Y* exists. For example, in the relation

ORDER_LINE (Order#, Item#, Quantity,
Unit_price, Extended_price,

Discounted_price)

each
tuple represents an item from an order with a particular quantity, and the
price per unit for that item. In this relation, (Quantity, Unit_price ) → Extended_price by the formula

Extended_price = Unit_price _{*}
Quantity.

Hence,
there is a unique value for Extended_price for
every pair (Quantity, Unit_price ), and thus it conforms to the
definition of functional dependency.

Moreover,
there may be a procedure that takes into account the quantity discounts, the
type of item, and so on and computes a discounted price for the total quantity
ordered for that item. Therefore, we can say

(Item#, Quantity, Unit_price ) → Discounted_price, or (Item#, Quantity, Extended_price) → Discounted_price.

To check
the above FD, a more complex procedure COMPUTE_TOTAL_PRICE may have
to be called into play. Although the above kinds of FDs are technically present
in most relations, they are not given particular attention during
normalization.

**4. Domain-Key Normal
Form**

There is
no hard-and-fast rule about defining normal forms only up to 5NF. Historically,
the process of normalization and the process of discovering undesirable
dependencies were carried through 5NF, but it has been possible to define
stricter normal forms that take into account additional types of dependencies
and constraints. The idea behind **domain-key
normal form** (**DKNF**) is to specify
(theo-retically, at least) the *ultimate
normal form* that takes into account all possible types of dependencies and
constraints. A relation schema is said to be in **DKNF** if all constraints and dependencies that should hold on the
valid relation states can be enforced simply by enforcing the domain constraints
and key constraints on the relation. For a relation in DKNF, it becomes very
straightforward to enforce all data-base constraints by simply checking that
each attribute value in a tuple is of the appropriate domain and that every key
constraint is enforced.

However,
because of the difficulty of including complex constraints in a DKNF relation,
its practical utility is limited, since it may be quite difficult to specify
general integrity constraints. For example, consider a relation CAR(Make, Vin#) (where Vin# is the vehicle identification
number) and another relation MANUFACTURE(Vin#, Country) (where Country is the
country of manufacture). A general constraint may be of the following form: *If the Make is either ‘Toyota’ or ‘Lexus,’
then the first character* *of the Vin#
is a ‘J’ if the country of manufacture is ‘Japan’; if the Make is ‘Honda’ or
‘Acura,’ the second character of the Vin# is a ‘J’ if the country of
manufacture is ‘Japan.’*

There is
no simplified way to represent such constraints short of writing a procedure
(or general assertions) to test them. The procedure COMPUTE_TOTAL_PRICE above is
an example of such procedures needed to enforce an appropriate integrity
constraint.

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Fundamentals of Database Systems : Database Design Theory and Normalization : Relational Database Design Algorithms and Further Dependencies : Other Dependencies and Normal Forms |

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