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An query string include a name/value pair. When the Web server sees a query string, it passes the information to ASP.NET for further processing.
The SqlDataSource control can pick out the value and use it as part of a SQL query.
In the SqlDataSource control’s Properties window, after selecting the SelectQuery property and clicking the ellipsis button in the right-hand column, in the QueryStringField box, type variable name in the Command and Parameter Editor and choosing QueryString from the parameter source drop-down list.