Gets and sets the SelectExpression property.
            This section describes Select, a function that takes query expressions similar to the standard SQL SELECT statement.
            See http://docs.amazonwebservices.com/AmazonSimpleDB/latest/DeveloperGuide/UsingSelect.html for more information
            on building expressions.
            
 Declaration Syntax
 Declaration Syntax| C# | 
public string SelectExpression { get; set; }