Question

We have a loop in SQL Server 2005 that loops around on a table getting each items parent until it gets to the top of the tree:

DECLARE @T Table
(
  ItemID INT NOT NULL PRIMARY KEY,
  AncestorID INT NULL 
)

Which has data like this:

ItemID | AncestorID
  1          2
  2          3
  3          4
  4          NULL

We have a loop that basically does this:

DECLARE @AncestorID INT
SELECT @AncestorID = 1
WHILE (@AncestorID IS NOT NULL)
BEGIN   
  --Do some work   
  SELECT @AncestorID = T.AncestorID   
  FROM @T t  
  WHERE T.ItemID = @AncestorID

  print @AncestorID
END

(Yes I know SQL is set based, and this is processing row by row, the “Do some work” needs to be done line by line for a reason).

This has always worked fine until today when we ended up in an endless loop. Turns out the cause was some wrong data:

ItemID | AncestorID
  1          2
  2          3

  4          NULL

ItemID 3 was deleted. The loop now never ends because AncestorID is never NULL – it stays at 3.

Is there anyway to rewrite the select statement to make @AncestorID null if the SELECT query returns 0 rows, or do I need to have a separate SELECT statement to count the records and some IF ELSE type logic?

Answer 1

Is there anyway to rewrite the select statement to make @AncestorID
null if the SELECT query returns 0 rows,

You can use an aggregate on T.AncestorID.

SELECT @AncestorID = min(T.AncestorID)   
FROM @T t  
WHERE T.ItemID = @AncestorID