How to correlate update with select query

Question

I want to update every row in the column SalesDifference20132015 in the table Stockitems based on the values in different columns in the same table. I know the SELECT query work however when I plug it into my update query I get the subquery returned more than 1 value error. How do i correlate my select query with my update?

UPDATE warehouse.StockItems
    SET SalesDifference20132015 =
    (SELECT (SELECT COUNT(*)
    FROM Sales.Orders o
    join sales.OrderLines ol ON o.OrderID = ol.OrderID
    WHERE ol.StockItemID = si.StockItemID AND YEAR(o.OrderDate) = 2013
    Group by ol.StockItemID)
    -
    (SELECT COUNT(*)
    FROM Sales.Orders o
    join sales.OrderLines ol ON o.OrderID = ol.OrderID
    WHERE ol.StockItemID = si.StockItemID AND YEAR(o.OrderDate) = 2015
    Group by ol.StockItemID)

    FROM Warehouse.StockItems si
    )

Answer 1

Something like this should work:

UPDATE si
SET SalesDifference20132015 = DIFF.Diff
FROM
    warehouse.StockItems si
    JOIN
        (
            SELECT DISTINCT
                StockItemID
                ,
                    (
                        SELECT COUNT(*)
                        FROM
                            Sales.Orders o
                            JOIN sales.OrderLines ol ON o.OrderID = ol.OrderID
                        WHERE ol.StockItemID = si.StockItemID AND YEAR(o.OrderDate) = 2013
                        GROUP BY ol.StockItemID
                    )
                        -
                    (
                        SELECT COUNT(*)
                        FROM
                            Sales.Orders o
                            JOIN sales.OrderLines ol ON o.OrderID = ol.OrderID
                        WHERE ol.StockItemID = si.StockItemID AND YEAR(o.OrderDate) = 2015
                        GROUP BY ol.StockItemID
                    ) Diff
            FROM Warehouse.StockItems si
        ) DIFF
    ON si.StockItemID = DIFF.StockItemID