Condition in a recursive query

Question

I have the following recursive table-valued function in MS SQL, in order to retrieve a hierarchy of objects from the database:

WITH tmpField (ParentNum, ChildNum, FieldNum, FieldDescr, Iteration) AS
(
  SELECT Field.ParentNum, Field.ChildNum, Field.FieldNum, Field.FieldDescr, 1
  FROM Field
  WHERE Field.ParentNum = @ParentNum

  UNION ALL 

  SELECT Field.ParentNum, Field.ChildNum, Field.FieldNum, Field.FieldDescr, tmpField.Iteration + 1
  FROM Field INNER JOIN 
  tmpField on Field.ParentNum = tmpField.ChildNum
)
SELECT DISTINCT ParentNum AS ParentNum, ChildNum AS ChildNum, FieldNum, FieldDescr
FROM tmpField

I want to modify it in the following way:

In the last iteration, when there are no more 'children', I want the ChildNum field to have the value of FieldNum. In all previous iterations, ChildNum should have the value of the ChildNum field, as it is now.

Can anyone suggest a method to achieve this, using the above query as a starting point?

Please note: despite its name, the field ChildNum does not reference any children of a row, but it should be interpreted as the identifier of that row.

Answer 1

when there are no more children it means that ChildNum is NULL so:

...

UNION ALL 

  SELECT Field.ParentNum, 
         COALESCE(Field.ChildNum, Field.FieldNum) ChildNum,
         Field.FieldNum,
         ...

---

EDIT: (following Daan comment)

ok, in that case, we could check on ChildNum 'children' count :

    ...

    UNION ALL 

      SELECT F1.ParentNum, 
             CASE WHEN (SELECT COUNT(1) 
                          FROM FIELD F2 
                         WHERE F2.ParentNum = F1.ChildNum) = 0 
                  THEN F1.FieldNum
                  ELSE F1.ChildNum
             END ChildNum,
             F1.FieldNum, F1.FieldDescr, tmpField.Iteration + 1
      FROM Field F1 INNER JOIN 
      tmpField on F1.ParentNum = tmpField.ChildNum

...

---

EDIT2:

let's move the check outside:

WITH tmpField (ParentNum, ChildNum, FieldNum, FieldDescr, Iteration) AS
(
  SELECT Field.ParentNum, Field.ChildNum, Field.FieldNum, Field.FieldDescr, 1
  FROM Field
  WHERE Field.ParentNum = @ParentNum

  UNION ALL 

  SELECT Field.ParentNum, Field.ChildNum, Field.FieldNum, Field.FieldDescr, tmpField.Iteration + 1
  FROM Field INNER JOIN 
  tmpField on Field.ParentNum = tmpField.ChildNum
)
SELECT DISTINCT ParentNum AS ParentNum, 
                CASE WHEN EXISTS (SELECT NULL 
                                    FROM Field f 
                                   WHERE tmpField.ChildNum = f.ParentNum) 
                      THEN tmpField.ChildNum
                      ELSE tmpField.FieldNum
                 END ChildNum,
                FieldNum,
                FieldDescr
FROM tmpField

Answer 2

This should return data you need. I removed iteration as you don't use it later

JOIN VERSION

;WITH tmpField (ParentNum, ChildNum, FieldNum, FieldDescr) AS
(
  SELECT f.ParentNum, f.ChildNum, f.FieldNum, f.FieldDescr
    FROM Field f
   WHERE f.ParentNum = @ParentNum
  UNION ALL 
  SELECT f.ParentNum, f.ChildNum, f.FieldNum, f.FieldDescr
    FROM Field f 
   INNER JOIN tmpField on f.ParentNum = tmpField.ChildNum
)
SELECT t.ParentNum AS ParentNum, 
       Case When p.ParentNum is Null 
            Then t.FieldNum 
            Else t.ChildNum 
        End AS ChildNum, 
       t.FieldNum, 
       t.FieldDescr
FROM tmpField t
Left Join (Select distinct ParentNum From Field) p on t.ChildNum=p.ParentNum

or

SUBQUERY VERSION (modified to use EXISTS instead of COUNT)

;WITH tmpField (ParentNum, ChildNum, FieldNum, FieldDescr) AS
(
  SELECT f.ParentNum, f.ChildNum, f.FieldNum, f.FieldDescr
    FROM Field f
   WHERE f.ParentNum = @ParentNum
  UNION ALL 
  SELECT f.ParentNum, f.ChildNum, f.FieldNum, f.FieldDescr
    FROM Field f 
   INNER JOIN tmpField on f.ParentNum = tmpField.ChildNum
)
SELECT t.ParentNum AS ParentNum, 
       Case When Exists(Select * from Field p Where t.ChildNum=p.ParentNum)
            Then t.ChildNum 
            Else t.FieldNum 
        End AS ChildNum, 
       t.FieldNum, 
       t.FieldDescr
FROM tmpField t