This is useful but the clunkyness of using FROM/WHERE vs JOIN ... USING has always annoyed me in Postgres, which they've based this on. The MySQL version is sort of what my brain expects.
UPDATE inventory
SET quantity = inventory.quantity - daily.amt
FROM
(SELECT sum(quantity) AS amt, itemId FROM sales GROUP BY 2) daily USING( itemId );
Otherwise it's kind of annoying when you end up joining to several tables in the update, but have to use the WHERE clause to join back to the main table.
UPDATE inventory
INNER JOIN (SELECT sum(quantity) AS amt, itemId FROM sales GROUP BY 2) AS daily USING (itemId)
SET quantity = quantity - daily.amt;