UPDATE brand AS b SET b.products_quantity = (SELECT COUNT(*) FROM product AS p WHERE b.id = p.brand_id)

UPDATE brand SET brand.products_quantity = p.num_products
FROM
(SELECT COUNT(*) FROM product WHERE brand.id = product.brand_id) p

ERROR:  invalid reference to FROM-clause entry for table "brand"
LINE 3: (SELECT COUNT(*) FROM product WHERE brand.id = product.brand...
                                            ^
HINT:  There is an entry for table "brand", but it cannot be referenced from this part of the query.

UPDATE accounts SET (contact_last_name, contact_first_name) =
    (SELECT last_name, first_name FROM salesmen
     WHERE salesmen.id = accounts.sales_id);