Chapter 7: Database Design and Development

isting departments in the Department table. However, as with primary keys, just having the correct fields in place doesn’t mean that our data is guaranteed to be correct.

For example, try setting the DepartmentID field for one of the employees to 123. SQL Server won’t mind making the change for you, so if you tried this in practice, you’d end up storing invalid data. However, after we set the foreign keys correctly, SQL Server will be able to ensure the integrity of our data—specifically, it will forbid us to assign employees to nonexistent departments, or to delete departments with which employees are associated.

The easiest way to create foreign keys using Visual Web Developer or SQL Server Management Studio is through database diagrams, so let’s learn about them.

Using Database Diagrams

To keep the data consistent, the Dorknozzle database really should contain quite a few foreign keys. The good news is that you have access to a great feature called database diagrams, which makes it a cinch to create foreign keys. You can define the table relationships visually using the database diagrams tool in Visual Web Developer or SQL Server Management Studio, and have the foreign keys generated for you.

Database diagrams weren’t created specifically for the purpose of adding foreign keys. The primary use of diagrams is to offer a visual representation of the tables in your database and the relationships that exist between them, to help you to design the structure of your database. However, the diagrams editor included in Visual Web Developer and SQL Server Management Studio is very powerful, so you can use the diagrams to create new tables, modify the structure of existing tables, or add foreign keys.

Let’s start by creating a diagram for the Dorknozzle database. To create a database diagram in Visual Web Developer, right-click the Database Diagrams node, and select Add New Diagram, as shown in Figure 7.15.

The process is similar in SQL Server Management Studio, which, as Figure 7.16 illustrates, has a similar menu.

The first time you try to create a diagram, you’ll be asked to confirm the creation of the database structures that support diagrams. Select Yes from the dialog, which should look like the one shown in Figure 7.17.

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Using Database Diagrams

Figure 7.15. Creating a database diagram with Visual Web Developer

Figure 7.16. Creating a database diagram with SQL Server Management Studio

Figure 7.17. Adding support for database diagrams

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Figure 7.18. Adding tables to the diagram

Next, a dialog like the one in Figure 7.18 will ask you which of your database tables you want included in the diagram. If you’re working with a database that comprises many tables, you may want to have diagrams built to represent specific pieces of functionality, but we want to create a diagram that includes all the tables in our database.

Click Add until all the tables are added to the diagram. As you click Add, the tables will be removed from the list and will appear in the diagram. Once you’ve added all the tables, click Close. You’ll see a window in which all the tables are clearly displayed—something like Figure 7.19.

You’ll probably need to tweak their positioning and dimensions so they fit nicely into the window. The zooming feature may prove useful here! Select File > Save Diagram1 (or similar) to save your new diagram. Enter Dorknozzle for the diagram’s name.

Now, if you right-click any table in the diagram, you’ll gain access to a plethora of possibilities, as Figure 7.20 reveals. This menu, along with the other diagramming features, are identical in Visual Web Developer and SQL Server Management Studio.

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Using Database Diagrams

Figure 7.19. Visualizing data tables using a diagram

Figure 7.20. The many features of the diagram editor

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Expanding the Table View submenu gives you more options for displaying your table. If you choose Standard, you’ll see a full-blown version of the table definition; as Figure 7.21 shows, you can change the table structure directly in the diagram! The diagraming features provided for free are extremely powerful and useful.

Figure 7.21. The standard table view

Implementing Relationships in the Dorknozzle

Database

Every table in the Dorknozzle database has a relationship with another table. To create a foreign key using the diagram, click the gray square to the left-hand side of the column for which you want to create the foreign key, and drag it over the table to which you want it to relate.

Let’s give it a try. Start by dragging the DepartmentID column of the Employees table over the DepartmentID column of the Departments table, as illustrated in Figure 7.22.

Figure 7.22. Creating a link between Employees and Departments

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The designer will open a dialog that shows the details of the new foreign key, like the one shown in Figure 7.23.

Figure 7.23. Adding a foreign key

Ensure that your data matches that shown in Figure 7.23, and click OK. A new dialog like the one shown in Figure 7.24 will appear, allowing you to tweak numerous options that relate to the new foreign key. Leave the default options as they are for now (though we’ll discuss them shortly), and click OK to finish up.

Figure 7.24. Editing the foreign key options

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After creating the foreign key, make a quick test to ensure that the relationship is indeed enforced. Try adding an employee, but set the person’s DepartmentID to 123. You should see an error like the one pictured in Figure 7.25.

Figure 7.25. The foreign key disallowing the addition of invalid data

If you tried to delete a department with which employees were associated, you’d generate a similar error.

Table 7.15 shows the foreign keys that we need to establish in the Dorknozzle database. In our project, the foreign key column has the same name as its corresponding primary key column. Go ahead and create all the foreign keys outlined in Table 7.15.

Table 7.15. The relationships in the Dorknozzle database

Primary Key Foreign Key

DepartmentID in the table Departments DepartmentID in the table Employees

EmployeeID in the table Employees	EmployeeID in the table HelpDesk
CategoryID in the table HelpDeskCat-	CategoryID in the table HelpDesk
egories
SubjectID in the table HelpDeskSub-	SubjectID in the table HelpDesk
jects
StatusID in the table HelpDeskStatus	StatusID in the table HelpDesk

When it’s complete, your relationship diagram should resemble Figure 7.26. After you add the relationships, save your changes by selecting File > Save Dorknozzle. When you’re asked to confirm the changes to the database tables you’re altering, click Yes.

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