Mysql
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Erzwingen aller Zeilen aus der ersten Tabelle eines Joins

SELECT p.*
     , SUM(mp.quantity) AS qty
FROM products p
  LEFT JOIN machine_products mp
    ON mp.product_id = p.product_id
    AND mp.machine_id = m              --- this condition moved from WHERE to ON
WHERE p.category_id = c
GROUP BY p.product_id