Using Subtypes and Supertypes in Software

Encode Code 128 Code Set B in Software Using Subtypes and Supertypes
Using Subtypes and Supertypes
Encoding Code 128 Code Set C In Visual C#.NET
Using Barcode generation for Visual Studio .NET Control to generate, create Code-128 image in Visual Studio .NET applications.
FIGURE 27 Simple subtype cluster own entities, because we offer cable broadband to residential and commercial customers, and we offer DSL only to residential customers Both cable and DSL could be stand-alone entities, but we wouldn t be seeing the entire relationship There are attributes in the BroadBand entity that we don t track in each of the child entities, and attributes in the child entities that we don t track in the BroadBand entity And we need to leave the design open to add more broadband types in the future without having to alter existing records To solve this problem, we designate BroadBand as a supertype, and the Cable and DSL entities as subtypes To do this, first we create the child entities with their specific attributes, without a primary key Then we create a required identifying relationship between the parent entity and each child entity; this relationship designates that the primary key from BroadBand be the primary key for each child Finally, we choose a discriminator, which is an attribute in the parent entity whose value determines which subtype a given record belongs to; the discriminator can be a key or non-key attribute In this case, our discriminator is Type, which contains a string value of either DSL or Cable If a subtype cluster contains all possible subtypes for the supertype for which they are defined, the subtype cluster is said to be complete Alternatively, if it includes only some of the possible subtypes, the cluster is incomplete The designation is mostly a documentation concern, but as
Code 128 Code Set A Generation In Visual Studio .NET
Using Barcode creator for ASP.NET Control to generate, create Code 128 Code Set C image in ASP.NET applications.
2
Print Code 128C In Visual Studio .NET
Using Barcode drawer for .NET framework Control to generate, create Code128 image in VS .NET applications.
Elements Used in Logical Data Models
Code 128 Encoder In Visual Basic .NET
Using Barcode creator for Visual Studio .NET Control to generate, create Code 128 Code Set A image in .NET applications.
with most design considerations, documenting the specifics can be helpful in the future for other developers working from this model Generally, physical implementation of a subtype cluster must be determined on a case-by-case basis Subtype clusters can be implemented in a one-to-one relationship of entities to tables, or some combination of tables and relationships The most important aspects to remember are the propagation of the primary key among all the entities, as well as constraints on the discriminator to ensure that all the records end up in the correct tables
Code-39 Generator In .NET
Using Barcode creator for .NET framework Control to generate, create Code 39 image in .NET framework applications.
When to Use Subtype Clusters
Data Matrix ECC200 Printer In VS .NET
Using Barcode printer for ASP.NET Control to generate, create Data Matrix 2d barcode image in ASP.NET applications.
Inevitably, every data model contains entities that contain attributes that hold information about a small subset of the records in the entity Whenever you find this happening in a data model, investigate further to see whether these attributes would be good candidates for a subtype cluster However, be careful not to try to force a supertype/subtype relationship; doing so leads to a confusing data model that has more entities than necessary Additionally, the existence of superfluous subtype clusters can lead to confusion in the physical implementation, often resulting in unnecessary tables and constraints This could ultimately lead to poor performance and the inability to maintain the database efficiently Subtype clusters can be a very powerful tool to build flexibility into a data model Because modeling data in this type of generalized hierarchy can allow future modifications without the need to change existing entities, searching for logical relationships where you can use subtype clusters should be considered time well spent
Scanning Code 3/9 In VS .NET
Using Barcode reader for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
Summary
Making Barcode In C#.NET
Using Barcode encoder for .NET framework Control to generate, create bar code image in .NET applications.
In this chapter, we ve covered the tools used to build a logical data model Every data model consists of the objects necessary to describe the data being stored, definitions of how individual pieces of data are related to one another, and any constraints that exist on that data Now that we ve covered the building blocks of the logical data model, 3 will help you bridge the gap between the logical objects and the physical implementation Later you will use this theory to build a data model for the Mountain View Music company
Barcode Creation In Java
Using Barcode printer for Java Control to generate, create bar code image in Java applications.
Code 128 Decoder In Visual Studio .NET
Using Barcode decoder for VS .NET Control to read, scan read, scan image in Visual Studio .NET applications.
Create EAN 13 In .NET Framework
Using Barcode generation for ASP.NET Control to generate, create GS1 - 13 image in ASP.NET applications.
DataMatrix Printer In Java
Using Barcode generation for Java Control to generate, create Data Matrix ECC200 image in Java applications.
Making Bar Code In Visual C#
Using Barcode maker for Visual Studio .NET Control to generate, create barcode image in .NET framework applications.
Painting Bar Code In VS .NET
Using Barcode maker for Visual Studio .NET Control to generate, create barcode image in VS .NET applications.
Bar Code Recognizer In .NET
Using Barcode reader for .NET Control to read, scan read, scan image in .NET framework applications.