| Data normalization is a process in which data attributes within a data model are organized to increase the cohesion of entity types. In other words, the goal of data normalization is to reduce and even eliminate data redundancy, an important consideration for application developers because it is incredibly difficult to stores objects in a relational database that maintains the same information in several places. Table 1 summarizes the three most common forms of normalization ( First normal form (1NF), Second normal form (2NF), and Third normal form (3NF)) describing how to put entity types into a series of increasing levels of normalization. Higher levels of data normalization are beyond the scope of this article. With respect to terminology, a data schema is considered to be at the level of normalization of its least normalized entity type. For example, if all of your entity types are at second normal form (2NF) or higher then we say that your data schema is at 2NF. |
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Level
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Rule
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An entity type is in 1NF when it contains no repeating groups
of data.
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An entity type is in 2NF when it is in 1NF and when all of
its non-key attributes are fully dependent on its
primary key.
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An entity type is in 3NF when it is in 2NF and when all of
its attributes are directly dependent on the
primary key.
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1. First Normal Form (1NF)
Let’s consider an example. n entity type
is in first normal form (1NF) when it contains no repeating groups
of data. For example, in
Figure
1 you see that there are several repeating attributes in the data Order0NF
table – the ordered item information repeats nine times and the contact
information is repeated twice, once for shipping information and once for
billing information. Although this initial version of orders could work, what
happens when an order has more than nine order items?
Do you create additional order records for them?
What about the vast majority of orders that only have one or two items?
Do we really want to waste all that storage space in the database for the
empty fields? Likely not.
Furthermore, do you want to write the code required to process the nine
copies of item information, even if it is only to marshal it back and forth
between the appropriate number of objects.
Once again, likely not.
Figure 1. An Initial Data
Schema for Order (UML
Notation).
| Figure 2 presents a reworked data schema where the order schema is put in first normal form. The introduction of the OrderItem1NF table enables us to have as many, or as few, order items associated with an order, increasing the flexibility of our schema while reducing storage requirements for small orders (the majority of our business). The ContactInformation1NF table offers a similar benefit, when an order is shipped and billed to the same person (once again the majority of cases) we could use the same contact information record in the database to reduce data redundancy. OrderPayment1NF was introduced to enable customers to make several payments against an order – Order0NF could accept up to two payments, the type being something like “MC” and the description “MasterCard Payment”, although with the new approach far more than two payments could be supported. Multiple payments are accepted only when the total of an order is large enough that a customer must pay via more than one approach, perhaps paying some by check and some by credit card. |
Figure 2. An Order Data
Schema in 1NF (UML
Notation).
An important thing to notice is the application of primary
and foreign keys in the new solution. Order1NF
has kept OrderID, the original key of Order0NF,
as its primary key. To
maintain the relationship back to Order1NF,
the OrderItem1NF table includes the OrderID
column within its schema, which is why it has the
stereotype of FK. When a new table is introduced into a schema, in this case OrderItem1NF,
as the result of first normalization efforts it is common to use the primary key
of the original table (Order0NF) as
part of the primary key of the new table. Because OrderID
is not unique for order items, you can have several order items on an order, the
column ItemSequence was added to form
a composite primary key for the OrderItem1NF
table. A different approach to keys
was taken with the ContactInformation1NF
table. The column ContactID, a surrogate key that has no business meaning, was made
the primary key.
2. Second Normal Form (2NF)
Although the solution presented in
Figure
2 is improved over that of Figure 1, it can
be normalized further. Figure 3 presents the data
schema of Figure 2 in second normal form (2NF). an entity type
is in second normal form (2NF) when it is in 1NF and when every
non-key attribute, any attribute that is not part of the primary key, is fully
dependent on the primary key. This
was definitely not the case with the OrderItem1NF table, therefore we need to introduce the new table Item2NF. The problem with OrderItem1NF
is that item information, such as the name and price of an item, do not depend
upon an order for that item. For
example, if Hal Jordan orders three widgets and Oliver Queen orders five
widgets, the facts that the item is called a “widget” and that the unit
price is $19.95 is constant. This
information depends on the concept of an item, not the concept of an order for
an item, and therefore should not be stored in the order items table –
therefore the Item2NF table was
introduced. OrderItem2NF
retained the TotalPriceExtended
column, a calculated value that is the number of items ordered multiplied by the
price of the item. The value of the
SubtotalBeforeTax column within the Order2NF
table is the total of the values of the total price extended for each of its
order items.
3. Third Normal Form (3NF)
An entity type is in third
normal form (3NF) when it is in 2NF and when all of its attributes are
directly dependent on the primary key. A better way to word this rule might be
that the attributes of an entity type must depend on all portions of the primary
key.
In this case there is a problem with the OrderPayment2NF
table, the payment type description (such as “Mastercard” or “Check”)
depends only on the payment type, not on the combination of the order id and the
payment type. To resolve this
problem the PaymentType3NF table was
introduced in Figure 4, containing a
description of the payment type as well as a unique identifier for each payment
type.
4. Beyond 3NF
The data schema of Figure 4
can still be improved upon, at least from the point of view of data redundancy,
by removing attributes that can be calculated/derived from other ones.
In this case we could remove the SubtotalBeforeTax
column within the Order3NF table and
the TotalPriceExtended column of OrderItem3NF,
as you see in Figure 5.
Figure 5. An Order Without
Calculated Values (UML
Notation).
5. Why Data Normalization?
The advantage of having a highly normalized data schema is that
information is stored in one place and one place only, reducing the possibility
of inconsistent data. Furthermore,
highly-normalized data schemas in general are closer conceptually to
object-oriented schemas because the object-oriented goals of promoting high
cohesion and loose coupling between classes results in similar solutions (at
least from a data point of view). This
generally makes it easier to map your objects to your data schema.
6. Denormalization
From a purist point of view you want to normalize your data
structures as much as possible, but from a practical point of view you will find
that you need to 'back out" of some of your normalizations for performance
reasons. This is called "denormalization". For example, with the data schema of Figure 1 all the
data for a single order is stored in one row (assuming orders of up to nine
order items), making it very easy to access.
With the data schema of Figure 1 you could
quickly determine the total amount of an order by reading the single row from
the Order0NF table.
To do so with the data schema of Figure
5 you would need to read data from a row in the Order
table, data from all the rows from the OrderItem
table for that order and data from the corresponding rows in the Item
table for each order item. For this
query, the data schema of Figure 1 very likely
provides better performance.
7. Acknowledgements
I'd like to thank Jon Heggland for his thoughtful review
and feedback. He found
several bugs which had gotten by both myself and my tech reviewers.
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