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In our example for this chapter, we re exploring ways to query our books to see those that cost less than $30 and group them by subject. To do this, we can separate the process into several separate tasks: selecting the ordered subjects, selecting the corresponding books filtered on price, combining the subjects with the books, and projecting only the results that we need. Let s start by looking at the book-related tasks first and then deal with joining them to the subjects. Listing 6.2 restates the book portion of our starting query.
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Listing 6.2 Selecting the book title and price for books less than $30
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IEnumerable<Book> books = Book.GetBooks(); var query = from book in books where book.Price < 30 orderby book.Title
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Getting started with LINQ to SQL
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select new { book.Title, book.Price };
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At this point, we re still requesting everything from the database and filtering it on the client. To prove this, here is the SQL statement sent to the database:
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SELECT FROM ID, Isbn, Notes, PageCount, Price, PubDate, Publisher, Subject, Summary, Title Book
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Throughout our discussion, the generated SQL we present is based on a prerelease version of Visual Studio 2008. Some of the query details may differ from the final release, but the basic concepts should still apply.
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We re still retrieving all of the fields from the database even though we re only using two of them. Additionally, we re fetching all records from the database, not just the ones that meet our criteria. Also, we re not leveraging our indexes because we re ordering the results on the client. Ideally, we d like to issue a statement to the database like the following:
SELECT FROM WHERE ORDER BY Title, Price Book AS t0 (Price < @p0) Title
How many changes does it take to our query to make this change None! All we need to do is modify our Book class and change how we re accessing it. Let s start by revisiting the Book object and table (shown in figure 6.1) to see what changes we will need to make.
Figure 6.1 Comparing the Book table with the Book class
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To begin, we re going to apply a 1-1 field mapping between the table and our destination object. Later in this chapter, we will take a look at joining this table with the corresponding subjects and see how we can handle the foreign key relationships in the database. For now, we will limit our focus to a single table. Let s start by looking at the code for the Book class, shown in listing 6.3.
Listing 6.3 Starting Book class definition
public class Book { public Guid BookId { get; set; } public String Isbn { get; set; } public String Notes { get; set; } public Int32 PageCount { get; set; } public Decimal Price { get; set; } public DateTime PublicationDate { get; set; } public String Summary { get; set; } public String Title { get; set; } public Guid SubjectId { get; set; } public Guid PublisherId { get; set; } }
For the sake of this discussion, we use the auto-implemented properties discussed in section 2.2.2. We re left with a clean class definition that includes only the public property declarations. At this point, we still need a way to fill our objects with data from the database. We do this by setting up a series of mappings to specify how our objects relate to the database tables and columns.
NOTE
Auto-implemented properties are not available in VB 9.0. With VB, you will need to explicitly include the private backing fields along with the get/set accessors.
Setting up the object mapping
Let s start our exploration of LINQ to SQL by enabling the Book class. To begin, add a reference to the System.Data.Linq assembly, which is part of the .NET 3.5 Framework, and add a using statement to the top of the class. The Mapping namespace contains attributes that enable us to declaratively establish the relationship between the database and objects.
using System.Data.Linq.Mapping;
For the sake of clarity, we will use attributes to declare our data mappings for this chapter. We ll only look at the basics at this point. In chapter 7, we will dive deeper into the mapping options, but for now we simply want to get it working.
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