Как упростить этот запрос, чтобы вместо двух использовать только один SELECT
и выводилось только одно поле warehouseName без максимального значения?

SELECT B.warehouseName, MAX(B.SUMquantity) FROM
              (SELECT warehouse.name warehouseName, SUM(goods.quantity) as SUMquantity
              FROM warehouse
              LEFT JOIN goods ON warehouse.id=goods.warehouseId
              GROUP BY warehouse.name)
AS B

Всё догадался
SELECT warehouse.name warehouseName, SUM(goods.quantity) as SUMquantity
FROM warehouse
LEFT JOIN goods ON warehouse.id=goods.warehouseId
GROUP BY warehouse.name
ORDER BY SUM(goods.quantity) DESC LIMIT 1

Как объединить результаты двух этих запросов?

(SELECT product.name productName, SUM(goods.quantity) as SUMquantity
FROM product
LEFT JOIN goods ON product.id=goods.productId
GROUP BY product.name
HAVING SUMquantity>100)

(SELECT warehouse.name warehouseName FROM warehouse
LEFT JOIN goods ON warehouse.id=goods.warehouseId
GROUP BY warehouse.name
ORDER BY SUM(goods.quantity) DESC LIMIT 1)