How to: Filter Query Results by Using LINQ (Visual Basic)

Language-Integrated Query (LINQ) makes it easy to access database information and execute queries.

The following example shows how to create a new application that performs queries against a SQL Server database and filters the results by a particular value by using the Where clause. For more information, see Where Clause (Visual Basic).

The examples in this topic use the Northwind sample database. If you do not have the Northwind sample database on your development computer, you can download it from the Microsoft Download Center Web site. For instructions, see Downloading Sample Databases (LINQ to SQL).

To create a connection to a database

1. In Visual Studio, open Server Explorer/Database Explorer by clicking Server Explorer/Database Explorer on the View menu.

2. Right-click Data Connections in Server Explorer/Database Explorer and then click Add Connection.

3. Specify a valid connection to the Northwind sample database.

To add a project that contains a LINQ to SQL file

1. In Visual Studio, on the File menu, point to New and then click Project. Select Visual Basic Windows Forms Application as the project type.

2. On the Project menu, click Add New Item. Select the LINQ to SQL Classes item template.

3. Name the file northwind.dbml. Click Add. The Object Relational Designer (O/R Designer) opens for the northwind.dbml file.

To add tables to query to the O/R Designer

1. In Server Explorer/Database Explorer, expand the connection to the Northwind database. Expand the Tables folder.

   If you have closed the O/R Designer, you can reopen it by double-clicking the northwind.dbml file that you added earlier.

2. Click the Customers table and drag it to the left pane of the designer. Click the Orders table and drag it to the left pane of the designer.

   The designer creates new Customer and Order objects for your project. Notice that the designer automatically detects relationships between the tables and creates child properties for related objects. For example, IntelliSense will show that the Customer object has an Orders property for all orders related to that customer.

3. Save your changes and close the designer.

4. Save your project.

To add code to query the database and display the results

1. From the Toolbox, drag a DataGridView control onto the default Windows Form for your project, Form1.

2. Double-click Form1 to add code to the Load event of the form.

3. When you added tables to the O/R Designer, the designer added a DataContext object for your project. This object contains the code that you must have to access those tables, in addition to individual objects and collections for each table. The DataContext object for your project is named based on the name of your .dbml file. For this project, the DataContext object is named northwindDataContext.

   You can create an instance of the DataContext in your code and query the tables specified by the O/R Designer.

   Add the following code to the Load event to query the tables that are exposed as properties of your data context. The query filters the results and returns only customers that are located in London.

   Dim db As New northwindDataContext
   
   Dim customers_London = From cust In db.Customers 
                          Where cust.City = "London"  
                          Select cust.CustomerID, cust.CompanyName, 
                                 OrderCount = cust.Orders.Count, 
                                 cust.City, cust.Country
   
   DataGridView1.DataSource = customers_London
   

4. Press F5 to run your project and view the results.

5. Following are some other filters that you can try.

   Dim companies_H = From cust In db.Customers 
                     Where cust.Orders.Count > 0 And
                           cust.CompanyName.StartsWith("H") 
                     Select cust.CustomerID, cust.CompanyName, 
                            OrderCount = cust.Orders.Count, 
                            cust.Country
   
   Dim customers_USA = From cust In db.Customers 
                       Where cust.Orders.Count > 15 And
                             cust.Country = "USA"  
                       Select cust.CustomerID, cust.CompanyName, 
                              OrderCount = cust.Orders.Count, 
                              cust.Country
   

See Also

Other Resources

LINQ in Visual Basic

Queries (Visual Basic)

LINQ to SQL

DataContext Methods (O/R Designer)

Walkthrough: Creating LINQ to SQL Classes (O/R Designer)