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Occasionally, when you're designing a database schema, you need to go to fifth normal form (5NF). Tables implemented in 5NF are often severely decomposed into many small tables to minimize redundancy and assist in day-to-day management (as I'll demonstrate in a moment). Complex multitable joins are necessary to recombine fragmented data into the consolidated information that most end users expect to see. For situations that require a 5NF database, you can minimize the performance impact of multitable joins by creating views of your data.
What Is a View?
A view is a way of pulling together data from a database into a virtual table. You create a view to give the user a specific informational perspective of the database instead of the static table perspective. For instance, you can use a view to answer a question such as "What types of birds are in Iowa in summer?" You can build a view on one or many tables to provide selected information rather than piles of raw data. You create a view with a CREATE VIEW statement, and—unlike ad hoc queries, which you execute from the ISQL window—the view's definition is stored in the database catalog. After you create the view, you can reference it by name in a simple SELECT statement. The benefit of using a view is that you can restrict, reorganize, or recombine data to present information, as I'll show in this article.
Normal Is Relative
This month, the example design is a bird-watchers' reference database. (An online version of this database is at http://www.petersononline.com/birds/.) Figure 1 is the entity relationship diagram (ERD) of the database. The central table is Bird. Tables that contain names (common and Latin), color, habitat, region, and seasonal data about each bird species relate to the Bird table. Each table is in 3NF: No repeating groups are in the table, all non-key columns are fully functionally dependent on the primary key, and no non-key column determines the value of a second non-key column. (See the sidebar on page 72 for detailed instructions on how to read the ERD.)
The normal form of each of these tables goes beyond 3NF, however. Within each table, each determinant is also a candidate key or else no candidates exist other than the primary key. (A determinant is a column that describes, identifies, limits, or otherwise determines the values of the other columns in a table. Each table has at least one determinant, the column or columns that make up the primary key.) This condition qualifies the entire set of tables for an advanced level of 3NF called Boyce-Codd Normal Form (BCNF). BCNF simply removes some modification anomalies that might result if a determinant is not a candidate key. For instance, Figure 1 includes the many-to-many (M:N) relationship of Bird to ColorNames, with the relation BirdColors associating these two tables. If you had just a one-to-many (1:M) relation, Bird to BirdColors, and if BirdColors contained the color text instead of the color identifiers, as in Figure 2, this version of BirdColors would be subject to a modification anomaly. For example, if you want to change the color text red to scarlet in the 3NF arrangement that Figure 2 represents, you have to update many rows. In a 5NF database schema such as in Figure 1, you simply update one row in one table. BCNF removes the modification anomaly of having to make a change in many places in the database when you want to change one value.
Figure 3, a logical representation of the ERD with sample data, demonstrates how fragmented this data is. This bird-watchers' reference database is available in four languages, so for maintenance and installation reasons, text and number data types are segregated into separate tables. Text data (colors, habitat descriptions, etc.) must be available in any language the installer selects (English, French, German, or Spanish), but number data (identifier, minimum and maximum size) looks the same in all these language selections. The installation package has to include a set of text tables for each of the languages supported. Storing the text data in separate base tables (for example, SeasonNames—two rows) rather than as part of the associative tables (for example, BirdRegions—28,400 rows) significantly reduces the amount of data in the installation package. Thus, the decision to decompose this database into 5NF results in ease of data maintenance and simplicity of installation.
Formulating Views
But the user of this database doesn't want to see the data in fragments. The user wants information such as "What kinds of birds can I expect to see in Iowa during the summer?" You can find that answer by formulating a view, as you see in Listing 1. A partial list of the view results appears in Table 1.
You can call the view directly with a SELECT statement, as in Listing 1, or you can write a stored procedure that calls the view, as in Listing 2. These two techniques are somewhat restrictive; the output of each is limited to birds you can see in Iowa in the summertime. To maximize your options (or your users' options), you can write a stored procedure like the one in Listing 3. This approach lets you retrieve the bird list from any season and any region, simply by feeding different arguments to the stored procedure when you execute it. This stored procedure has a query plan that is evaluated, compiled, and stored. Each time the stored procedure is called, the query plan is referenced and executed for optimal performance.
In SQL Server 7.0, when the structure of a table that is used by a stored procedure changes, the stored procedure automatically recompiles. Also, according to my findings on totally unindexed tables, all three of these code options run in about the same amount of time, after the data is in cache.
In SQL Server 6.5, the view and stored procedure names and code are stored in sysobjects; the query trees are stored in sysprocedures. SQL Server 7.0 treats the stored procedures more like views, and only the code is stored in sysobjects; each time SQL Server starts, the query plan is recalculated and kept in procedure cache. Column information is stored in syscolumns, and any dependencies are stored in sysdepends. Text documenting the views is stored in syscomments along with the stored procedures.
Get It All Together
When you need to design and implement a database in 5NF, you're not limited to looking at data in fragmented bits. You can use views and stored procedures to organize and recombine the data into recognizable form. Creating and using views and stored procedures is a good way to extract information from a highly normalized database.
