>Also show here your union statement. May be I will have an idea.

You asked for it and here it is. Remember before I was dealing with a concept SQL what follows is the real one that is erroring out.

SELECT ActionItem.ActionKey, ActionItem.ActionDate, ActionItem.ActionNumber,
       ActionItem.ActionAltId, ActionItem.DueDate, ActionItem.RestrictToUser,
       ActionItem.ActionCompleteDate, ActionItem.StatusKey, 
       ActionItem.AssignByUserKey, ActionItem.CategoryKey, 
       ActionAssignTo.ActionAssignToKey, ActionAssignTo.AssignToDueDate, 
       ActionAssignTo.AcceptRejectDelegate, ActionAssignTo.AssignCompleteDate, 
       ActionAssignTo.PriorityKey, ActionAssignTo.StatusKey, 
       ActionAssignTo.AssignToUserKey, ActionStatus.Status AS ActionStatus, 
       ActionCategory.CategoryName, ActionPriority.Priority, 
       AssignStatus.Status AS AssignStatus, 
       ActionUsers.FirstName AS ActionUserFname, 
       ActionUsers.lastName AS ActionUserLname, 
       AssignUsers.FirstName AS AssignUserFname, 
       AssignUsers.lastName AS AssignUserLname 
FROM ActionItem, ActionAssignTo, ActionStatus, ActionCategory, 
     users ActionUsers, ActionPriority, AssignStatus, users AssignUsers 
WHERE ActionItem.ActionKey = ActionAssignTo.ActionKey AND 
      ActionItem.StatusKey = ActionStatus.StatusKey AND 
      ActionItem.CategoryKey = ActionCategory.CategoryKey AND 
      ActionItem.AssignByUserKey = ActionUsers.UserKey AND 
      ActionAssignTo.PriorityKey = ActionPriority.PriorityKey AND 
      ActionAssignTo.StatusKey = AssignStatus.StatusKey AND 
      ActionAssignTo.AssignToUserKey = AssignUsers.UserKey AND 
      ActionItem.AssignByUserKey = 17 
UNION 
SELECT ActionItem.ActionKey, ActionItem.ActionDate, ActionItem.ActionNumber, 
       ActionItem.ActionAltId, ActionItem.DueDate, ActionItem.RestrictToUser, 
       ActionItem.ActionCompleteDate, ActionItem.StatusKey, 
       ActionItem.AssignByUserKey, ActionItem.CategoryKey, 
       0 AS ActionAssignToKey, ' ', ' ', ' ', 0, 0, 0, ' ', ' ', ' ', ' ', 
       ActionUsers.FirstName AS ActionUserFname, 
       ActionUsers.lastName AS ActionUserLname, ' ', ' ' 
FROM ActionItem, ActionStatus, ActionCategory, users ActionUsers 
WHERE ActionItem.ActionKey NOT IN (select ActionAssignTo.ActionKey from ActionAssignTo) AND 
      ActionItem.StatusKey = ActionStatus.StatusKey AND 
      ActionItem.CategoryKey = ActionCategory.CategoryKey AND 
      ActionItem.AssignByUserKey = ActionUsers.UserKey AND 
      ActionItem.AssignByUserKey = 17 
ORDER BY 1, 11