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Question 20-1. What makes a relational database different from any other kind of database Question 20-2. What s a primary key Question 20-3. What s a foreign key Question 20-4. Imagine a fictitious database for a bookseller. What query would you use to retrieve the contents of the Title column in the Books table
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20: ADO.NET and Relational Databases
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Question 20-5. In the same fictitious database, what query would you use to retrieve the contents of the Author column where the value in the Publisher column is OReilly Question 20-6. Why would you want to use a join Question 20-7. What .NET class represents a set of data retrieved from the database Question 20-8. What s the most common way to view the rows in a DataTable object Question 20-9. What s the purpose of the DataAdapter class Question 20-10. What method of the DataAdapter class do you use to provide the DataSet with the retrieved data
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Test Your Knowledge: Exercises
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Exercise 20-1. Let s start with a simple exercise. The Northwind database contains a table named Orders. Write a program to retrieve the order date and shipped date of all the records in the Orders table. Exercise 20-2. We ll try something slightly more complicated now. Write a program to display the name and ID of products with fewer than 10 units in stock. Exercise 20-3. Now for an exercise that involves multiple tables. Write a program to display the first and last names of the employees in region 1.
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As you saw in 20, SQL is a powerful tool for retrieving and filtering information from a database. Once you become accustomed to the syntax, with its selects and froms and joins, it s somewhat intuitive as well. However, SQL commands don t integrate well with C#, as you saw. You need the bridge of DataAdapter and DataSet objects to connect the database query with your application. The Language Integrated Query (LINQ) is the solution to that problem. LINQ is a new feature of .NET that C# 3.0 takes advantage of, which makes it easier to work with data, as you ll see in the second part of the chapter. Another useful feature of LINQ is that you can address a number of different data sources using similar syntax. In this chapter we ll show you how to use LINQ with SQL, but you don t need to use LINQ with a traditional database it can retrieve data from XML files and other data sources equally well. Perhaps the most interesting feature of LINQ is that you can query more than just data stored in other files. You can use LINQ to query collections that are held inmemory, that is, collection classes within your own code. So, for example, if you have a collection of Book objects, you can use LINQ to query for all the books by a single author, or published after a certain date. You could certainly write C# code to accomplish that, but the query syntax is arguably more natural and certainly briefer. Because this use of LINQ is easy to understand and is potentially useful, we ll start with that, and then move on to using it with a SQL database.
Querying In-Memory Data
As you ve seen elsewhere in this book, C# allows you to create classes that are complex, with many different properties, which sometimes are objects of other classes as well. You ve also seen how to create collections of objects that you can manipulate in different ways. Sometimes that complexity works against you, though. Suppose you have a class that represents shipping orders for a warehouse. You could keep a ton of
data in such an object, which would make it very versatile, but what if you just wanted to extract a list of the zip codes where your customers live, for demographic purposes You could write some code to go through the entire collection of objects and pull out just the zip codes. It wouldn t be terribly difficult, but it might be timeconsuming. If that information were in a database, you could issue a simple SQL query, like you learned about in 20, but collections can t be queried like a database...until now. Using LINQ, you can issue a SQL-like query against a collection in your code to get another collection containing just the data you want. An example will help make this clear. Before you can start, you ll need a collection to work with, so we ll define a quick and simple Book class, like so:
public class Book { public string Title { get; set; } public string Author { get; set; } public string Publisher { get; set; } public int PublicationYear { get; set; } }
This is a very basic class, with three string fields and one int field. Next, we ll define a generic List<Book>, and fill it with a handful of Book objects. This is a relatively short list, and it wouldn t be that hard to sort through by hand, if you needed to. That s because we re keeping the List short for demonstration purposes; in other cases, it might be a list of hundreds of items read in from a file or someplace else:
List<Book> bookList = new List<Book> { new Book { Title = "Learning C# 3.0", Author = "Jesse Liberty", Publisher = "O'Reilly", PublicationYear = 2008 }, new Book { Title = "Programming C# 3.0", Author = "Jesse Liberty", Publisher = "O'Reilly", PublicationYear = 2008 }, new Book { Title = "C# 3.0 Cookbook", Author = "Jay Hilyard", Publisher = "O'Reilly", PublicationYear = 2007 }, new Book { Title = "C# 3.0 in a Nutshell", Author = "Ben Albahari", Publisher = "O'Reilly", PublicationYear = 2007 }, new Book { Title = "Head First C#", Author = "Andrew Stellman", Publisher = "O'Reilly", PublicationYear = 2007 },
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