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Question

UPDATE QITEM 
   SET QTYONHAND = QTYONHAND + (SELECT SUM(DELQTY)
                                  FROM QDEL 
                                 WHERE QITEM.ITEMNAME = QDEL.ITEMNAME);

I am trying to update an inventory column where the intial values are all 1000 and I am adding the number of deliveries from the QDEL table to that 1000. This works but there are null values present in the rows with no deliveries in the QDEL table. I cannot figure out why???

Answer 1

Because SUM returns NULL if all DELQTY rows are NULL . You can do something like this :

UPDATE QITEM 
SET QTYONHAND = QTYONHAND + COALESCE((SELECT SUM(DELQTY)
FROM QDEL 
WHERE QITEM.ITEMNAME = QDEL.ITEMNAME), 0);

Answer 2

You need to add WHERE EXISTS so you're only updating rows where there is at least one corresponding record in QDEL :

UPDATE qitem qi
   SET qi.qtyonhand = qi.qtyonhand + ( SELECT SUM(qd.delqty)
                                         FROM qdel qd
                                        WHERE qd.itemname = qi.itemname )
 WHERE EXISTS ( SELECT 1 FROM qdel qd
                 WHERE qd.itemname = qi.itemname );

PS If it's possible for the value of delqty in QDEL to be NULL, you'll want to use SUM(NVL(qd.delqty, 0)) (or SUM(COALESCE(qd.delqty, 0)) ) rather than SUM(qd.delqty) .

Answer 3

Use NVL to always return a sum, even if no deliveries have taken place.

UPDATE QITEM 
SET QTYONHAND = QTYONHAND + (SELECT NVL(SUM(DELQTY),0)
    FROM QDEL 
    WHERE QITEM.ITEMNAME = QDEL.ITEMNAME);