Gets and sets the Exists property.
            Check for the existence of the attribute used for the conditional check.
            If this field is unspecified or true, UpdateCondition.Value must be set to a value.
            If this field is false, UpdateCondition.Value should not have a value assigned to it.
            If no value is associated with this field, SimpleDB uses a default value of 'true'
            
 Declaration Syntax
 Declaration Syntax| C# | 
public bool Exists { get; set; }