SQL Server sum and subtract prevent null results - sql

Question

I'm using this code:
SELECT
(SELECT SUM(TrnQty) AS Total
FROM InventoryTrans
WHERE InventoryItemID = (select MAX(InventoryItemID)
from inventorymaster)
AND CustomerID = '0') -
(SELECT SUM(TrnQty) AS Total
FROM InventoryTrans
WHERE InventoryItemID = (select MAX(InventoryItemID)
from InventoryMaster)
AND CustomerID > '0'
)
If the result of one them is NULL i take totally result NULL. How can I SUM if one of them is Null? Should I use CASE or something?

Answer 1

Here is a correct way to do it
SELECT Sum(CASE
WHEN customerid = '0' THEN trnqty
ELSE 0
END) - Sum(CASE
WHEN customerid > '0' THEN trnqty
ELSE 0
END) AS Total
FROM inventorytrans
WHERE inventoryitemid = (SELECT Max (inventoryitemid)
FROM inventorymaster)
EDIT:
I [#GordonLinoff] am providing a slightly simpler version of this answer. The use of conditional aggregation is correct, but there is a simpler way of writing it:
SELECT SUM(CASE WHEN it.customerid = '0' THEN it.trnqty
WHEN it.customerid > '0' THEN - it.trnqty
ELSE 0
END) as Total
FROM inventorytrans it
WHERE it.inventoryitemid = (SELECT Max(im.inventoryitemid)
FROM inventorymaster im
)

Answer 2

use
SELECT coalesce(SUM(TrnQty), 0) ...

Answer 3

You can also use
SELECT ISNULL(SUM(TrnQty),0)...

Answer 4

Simply wrap your existing inner select statements in an ISNULL function. It is simple to implement and your intent is clear, if your SELECT returns a null value, use 0.
SELECT
IsNull(
(
SELECT SUM(TrnQty) AS Total
FROM InventoryTrans
WHERE InventoryItemID =
(
select MAX (InventoryItemID) from inventorymaster
)
and CustomerID='0'
),0) -
IsNull(
(
SELECT SUM(TrnQty) AS Total
FROM InventoryTrans
WHERE InventoryItemID =
(
select MAX (InventoryItemID) from InventoryMaster
)
and CustomerID > '0'
),0)
Is this a great example of SQL, not really, but you can see how wrapping the select statements with IsNull is a minor change to your existing query to give you the results you need.
This query could be re-written into a single set based operation, here's my go at it:
SELECT SUM(CASE customerid WHEN '0' THEN -TrnQty ELSE TrnQty END) AS Total
FROM InventoryTrans
WHERE InventoryItemID =
(
select MAX (InventoryItemID) from inventorymaster
)
It is not necessary in this new query to check for null, (either with IsNull or Coalesce) unless you need to ensure that the end result is not null, this query will only return null if ALL of the TrnQty fields are also null. If that is a problem for you logic, wrap the SUM function in an IsNull function as well.