Imbedding A Select Statement Within An Update

I have the following list of fields in a table named call.


CallID, CallGroupID, PersonID, ParentID,


I have two records.


23, 45, John, Null
26, <Incorrect Value>, Jane, 23


Now, I'm wanting to set the CallGroupID of record two, to that of the
CallGroupID of record one.


This is what I came up with, but it doesn't work.


update call set CallGroupID = (Select CallGroupID from call where
CallID = ParentID) where ParentID is not null


As you know the Select statment I have embedded is trying to pull up a
list CallGroupIDs from records where the CallID = ParentID. What I
want it to do is pull up the CallGroupID of the record where the CallID

= ParentID.


The CallID is a unique field, so there is only one such record in the
table.


Any help?


Thanks