CAML query for null lookup field values

Question

I have a problem with filtering list items using CAML

I Have this list: Code - Text, Name - Text, Link - Text, Order No - Number, Parent - Lookup to this List. I want to filter all items where Parent is not selected (i.e none)

Here is my CAML query but it's not giving the right solution:

string camlquery = String.Format(@"<where>
                                                <Eq><FieldRef Name='{0}' /><Value Type='Lookup'></Value></Eq>
                                                </where>
                                               <OrderBy>
                                                <FieldRef Name='Order No'></FieldRef>
                                                </OrderBy>
                                               ", this.MenuItemParentField);

MenuItemParentField it's the name of the lookup column MenuItemParentField="Parent"

What is going wrong with my caml query

Any help would be appreciated

Thank you

Answer 1

Use IsNull Element (Query) instead of Eq Element (Query)

string camlquery = String.Format(@"<where> 
                                     <IsNull><FieldRef Name='Parent' /></IsNull> 
                                   </where> 
                                   <OrderBy> 
                                     <FieldRef Name='Order No'></FieldRef> 
                                   </OrderBy> 
                                   ", this.MenuItemParentField);