You can use LINQ to query and retrieve data from SQL table that our Silverlight Application can use. The following link contains an article with details instruction:
Also I would recommend you to take a look to the following RadMap control examples:
1. Items Virtualization example (http://demos.telerik.com/silverlight/#Map/DynamicLayer). The Using Dynamic Layer help topic of our documentation you can use also -- http://www.telerik.com/help/silverlight/map-using-dynamic-layer.html.
2. DataBinding example (http://demos.telerik.com/silverlight/#Map/DataBinding). It demonstrates using of ItemsSource property for information layer. The data binding help topic you can use also -- http://www.telerik.com/help/silverlight/map-data-binding.html.
All the best,
the Telerik team
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