T-SQL Query - Return One Result, or return 0 (SQL 2008R2) - sql

Question

I'm quite the novice when it comes to T-SQL, and I can't figure out this query.
I am trying to compare an Item's value to it's ledger Value to check for any anomalies, because occasionally they go out of sync.
I need the query to, for all current stock, subtract the Item Ledger's 'Amount' field from the Item's 'Item Value' field. There are several Item Ledger Entries, it must subtract the amount from the ledger entry that is Open. If there is no Open Ledger Entry, subtract zero instead.
I've butchered together a query in the query designer, apologies for the sloppiness:
SELECT dbo.[Company$ItemTable].[Item No_],
dbo.[Company$ItemTable].[Current Stock],
dbo.[Company$ItemTable].[Item Value],
dbo.[Company$Item Ledger Entry].[Open],
dbo.[Company$ItemTable].[Item Value] - SUM(CASE
WHEN dbo.[Company$Item Ledger Entry].[Open] = 1 THEN dbo.[Company$Item Ledger Entry].Amount
ELSE 0
END) AS Diff,
SUM(CASE
WHEN dbo.[Company$Item Ledger Entry].[Open] = 1 THEN dbo.[Company$Item Ledger Entry].Amount
ELSE 0
END) AS AmountILE,
dbo.[Company$Item Ledger Entry].[Entry No_]
FROM dbo.[Company$ItemTable]
LEFT OUTER JOIN dbo.[Company$Item Ledger Entry]
ON dbo.[Company$ItemTable].[Item No_] = dbo.[Company$Item Ledger Entry].[Lot No_]
GROUP BY dbo.[Company$ItemTable].[Item No_],
dbo.[Company$ItemTable].[Current Stock],
dbo.[Company$ItemTable].[Item Value],
dbo.[Company$Item Ledger Entry].[Open],
dbo.[Company$Item Ledger Entry].[Entry No_]
HAVING ( dbo.[Company$ItemTable].[Current Stock] = 1 )
AND ( dbo.[Company$ItemTable].[Item Value] - SUM(CASE
WHEN dbo.[Company$Item Ledger Entry].[Open] = 1 THEN dbo.[Company$Item Ledger Entry].Amount
ELSE 0
END) > 1 )
AND ( dbo.[Company$Item Ledger Entry].[Open] = 1 )
Does this make sense? Can anyone help? Thanks!

Answer 1

Your table structure is unclear. I can guess what you mean.
Is dbo.[Company$ItemTable].[Item No_] unique in table?
Is dbo.[Company$ItemTable] relates to dbo.[Company$Item Ledger Entry] as one-to-many?
If yes then
SELECT
items.[Item No_],
items.[Current Stock],
items.[Item Value],
items.[Item Value] - ISNULL( le.OpenAmount, 0) AS Diff,
ISNULL( le.OpenAmount, 0) AS AmountILE
FROM dbo.[Company$ItemTable] items
OUTER APPPLY
(
SELECT SUM(e.Amount)
FROM dbo.[Company$Item Ledger Entry] e
WHERE items.[Item No_] = e.[Lot No_]
AND e.[Open] = 1
) le(OpenAmount)
WHERE items.[Current Stock] = 1