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Two other extensions have been added to the base binding class in Silverlight 4: the TargetNullValue and FallBackValue extensions. These extensions allow you to specify data that will be displayed in the case when the data being bound to the control is not as expected. TargetNullValue provides a value for the binding that should be used if the data value being bound is null. An example of using this extension is shown here: <TextBox Text="{Binding ValueName, TargetNullValue='Value is Null'}" /> FallBackValue provides a value to be used if the bound data is missing or if there was a mismatch that occurred. An example of using the FallBackValue extension is shown here: <TextBox Text="{Binding ValueName, FallbackValue='Value Not Found'}" />

As mentioned previously, a table can have only one primary key. However, there may be another key that could just as easily be defined as a primary key. This is known as a candidate key, as it is a candidate for being the primary key. There is no logical difference at all between the definition of a candidate key and a primary key. For example, if we have a table that holds spare parts for a General Motors (GM) vehicle, we could have an internal GM part number to use when ordering parts at the head office for various GM branches. This part number would be unique and would likely be used as the primary key. However, a part number is also created by each of the manufacturers, which is unique to them. This, too, could be a primary key if we include the supplier identifier in the database. We can t have two primary keys, and we ve chosen the GM part number as the primary key, but we could create a candidate key using the manufacturer identifier and part number.

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So, you know that Rails generated some tests automatically. Let s open one of them now and take a look. We ll start with the Event test, located in test/unit/event_test.rb, as shown in Listing 9-1.

This is where having a constraint defined will ensure that unique values can only be entered into columns defined within the constraint. This sounds very much like the previous primary key definition, but there are differences. A unique constraint is not a primary key, but the column or columns defined within the constraint could be a primary key. Also, a unique constraint can contain NULL values, but recall that a primary key cannot. However, NULL is treated as any other value within a column; therefore, the columns used to make a unique constraint must remain unique, including the NULL value, when you re looking to insert or update data. Finally, it is possible to have multiple unique constraints, but you can have only one primary key.

require File.dirname(__FILE__) + '/../test_helper' class EventTest < Test::Unit::TestCase fixtures :events

Summary

A relationship in a SQL Server database is a logical link between two tables. It is impossible to have a physical link, although, as you will see later, a physical line is drawn between two tables when designing the database. To have a physical link would mean the actual data linking the two tables would be stored only once in a central location, and that information within the keys linking the tables would be stored more than once, which is just not the case. When defining a logical relationship, we re informing SQL Server that we ll be linking a primary key from the master table to a foreign key in another table. So already there is a need for two keys: one on each table. The following sections present specific details about relationships, starting with a look at how relationships work with the concept of referential integrity.

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