Answers to Quizzes and Exercises in Visual C#

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Appendix: Answers to Quizzes and Exercises
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The output looks like this, if you want to check your results:
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Products with less than ProductID: 5 Product ProductID: 8 Product ProductID: 17 Product ProductID: 21 Product ProductID: 29 Product ProductID: 31 Product ProductID: 32 Product ProductID: 45 Product ProductID: 53 Product ProductID: 66 Product ProductID: 68 Product ProductID: 74 Product 10 units in stock: Name: Chef Anton's Gumbo Mix Name: Northwoods Cranberry Sauce Name: Alice Mutton Name: Sir Rodney's Scones Name: Th ringer Rostbratwurst Name: Gorgonzola Telino Name: Mascarpone Fabioli Name: Rogede sild Name: Perth Pasties Name: Louisiana Hot Spiced Okra Name: Scottish Longbreads Name: Longlife Tofu
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(Somebody needs to order more of Chef Anton s Gumbo Mix.) If you want to check your results against what s in the database itself, right-click on the Products table in Database Explorer and select Show Table Data. Solution to 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. This exercise involves the join keyword, and requires you to look around in the tables a bit. If you open the Region table, you ll find the RegionID column, which is the value you want to compare. But the Region table doesn t mention EmployeeIDs. The Employees table has a column EmployeeID, which is a good start, and a column Region, but the values there don t match up with any columns in the Region table. Instead, you need to look at the EmployeeTerritories table. You can join Employees to EmployeeTerritories on the EmployeeID column. The only other column in the EmployeeTerritories table is the TerritoryID column, which is the foreign key for the Territories table. In the Territories table, you ll find the TerritoryID column, and aha! the RegionID column. So, you can craft a query like this:
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string commandString = "select e.FirstName, e.LastName "+ "from Employees e "+ "join EmployeeTerritories et on e.EmployeeID = et.EmployeeID "+ "join Territories t on et.TerritoryID = t.TerritoryID "+ "join Region r on t.RegionID = r.RegionID "+ "where r.RegionID = 1";
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The where clause checks for the employees in region 1, and the three join clauses chain back up to the Employees table, where you select the first and last name fields. The full code for this exercise is found in Example A-61.
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20: ADO.NET and Relational Databases |
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Example A-61. The solution to Exercise 20-3
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using using using using using using System; System.Collections.Generic; System.Linq; System.Text; System.Data; System.Data.SqlClient;
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namespace Exercise_20_3 { class Program { static void Main(string[] args) { // create the data connection string connectionString = "server=.\\sqlexpress; " + "Trusted_Connection=yes;database=Northwind"; // create the string to hold the SQL command // to get records from the Customers table string commandString = "select e.FirstName, e.LastName " + "from Employees e "+ "join EmployeeTerritories et on " + "e.EmployeeID = et.EmployeeID " + "join Territories t on et.TerritoryID = " + "t.TerritoryID "+ "join Region r on t.RegionID = " + "r.RegionID " + "where r.RegionID = 1"; // create the data adapter with the // connection string and command SqlDataAdapter myDataAdapter = new SqlDataAdapter(commandString, connectionString); // Create and fill the DataSet object DataSet myDataSet = new DataSet( ); myDataAdapter.Fill(myDataSet); // Retrieve the Orders table DataTable myDataTable = myDataSet.Tables[0]; // iterate over the rows collection // and output the fields Console.WriteLine("Employees in Region 1:"); foreach (DataRow dataRow in myDataTable.Rows) { Console.WriteLine("{0} {1}", dataRow["FirstName"], dataRow["LastName"]); } }
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Appendix: Answers to Quizzes and Exercises
Example A-61. The solution to Exercise 20-3 (continued)
} }
21: LINQ
Quiz Solutions
Solution to Question 21-1. LINQ allows you to query several different types of data sources, including SQL Server databases, XML files, and in-memory collections. Solution to Question 21-2. A LINQ query returns a collection that implements IEnumerable. The type of the objects in the collection is irrelevant. Solution to Question 21-3. The select keyword returns the result of a LINQ query. Solution to Question 21-4. You don t need to use any special syntax to return a complex type from a LINQ query. The compiler can infer the type, even if it s unnamed. Solution to Question 21-5. The range variable in a LINQ query doesn t have to be any type; it just has to be a valid C# name. The compiler will infer its type. Solution to Question 21-6. The lambda expression in a LINQ query returns a method used to evaluate the data set. That data is projected onto the range variable. Solution to Question 21-7. When you use LINQ to SQL, you need to add a reference to the System.Data.Linq namespace, not the System.Linq namespace that s added by default and supports all the basic LINQ functions. Solution to Question 21-8. Use the [Table] attribute, with the Name of the table to define a class as representing a SQL table. Solution to Question 21-9. To add table classes in the Object Relational Designer, you must establish a connection to the database, and then simply drag the tables onto the design surface. The classes will be generated for you automatically. Solution to Question 21-10. The constructor of the data context class using the Object Relational Designer doesn t require any parameters; it s generated automatically.
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