SQL Server 2017

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CREATE TABLE #TempDiff (
    SKU VARCHAR(50),
    Diff INT
);

INSERT INTO #TempDiff (SKU, Diff)
VALUES
('PROD1', 3),
('PROD2', 4),
('PROD3', 5);

CREATE TABLE Products (
    PK INT,
    ItemNumber VARCHAR(50)
);

INSERT INTO Products (PK,ItemNumber)
VALUES
(1,'PROD1'),
(2,'PROD2'),
(3,'PROD3'),
(4,'PROD4'),
(5,'PROD5');

CREATE TABLE Storage (
    StorageID INT,
    WarehouseID INT
);

INSERT INTO Storage (StorageID, WarehouseID)
VALUES
(1, 1),
(2, 1),
(3, 2);

CREATE TABLE StorageContents (
    StorageID INT,
    PK INT,
    Qty INT,
	PickingLocation BIT
);

INSERT INTO StorageContents (StorageID, PK, Qty, PickingLocation)
VALUES
(1, 1, 10, 0),
(1, 2, 20, 1),
(2, 1, 30, 0),
(2, 4, 50, 1),
(2, 5, 60, 1),
(2, 5, 70, 0),
(2, 10, 80, 1),
(3, 1, 0, 1);

select *
from StorageContents;

WITH productPickLocation AS (
    SELECT tpc.*
    FROM StorageContents tpc
    LEFT JOIN Storage tp ON tpc.StorageID = tp.StorageID
    WHERE PickingLocation = 1 AND tp.WarehouseID = 1
)
UPDATE StorageContents
SET Qty = StorageContents.Qty - td.Diff
FROM #TempDiff td
INNER JOIN Products pr ON td.SKU = pr.ItemNumber
LEFT JOIN productPickLocation ppl ON pr.pk = ppl.pk

SELECT * FROM StorageContents;

StorageID	PK	Qty	PickingLocation
1	1	10	False
1	2	20	True
2	1	30	False
2	4	50	True
2	5	60	True
2	5	70	False
2	10	80	True
3	1	0	True

StorageID	PK	Qty	PickingLocation
1	1	7	False
1	2	15	True
2	1	27	False
2	4	45	True
2	5	57	True
2	5	65	False
2	10	77	True
3	1	-5	True