equivalent postgresql syntax

-- MS SQL way
UPDATE Item
    SET Qty = ISNULL(X.SumQty,0)
FROM Item
LEFT JOIN
(
    SELECT        ItemCode, SUM(TranItem.Qty) AS SumQty
    FROM          TranItem
    GROUP BY      ItemCode
) AS X
ON X.ItemCode = Item.ItemCode


-- Postgres way
UPDATE     Item
SET     Qty = X.SumQty
FROM
(
    SELECT         Item.ItemCode, COALESCE(SUM(TranItem.Qty),0) AS SumQty
    FROM           Item
    LEFT JOIN      TranItem
    ON             TranItem.ItemCode = Item.ItemCode
    GROUP BY       Item.ItemCode
) AS X
WHERE X.ItemCode = Item.ItemCode