MySQL：異なるテーブルに同じ名前の左結合と列

フィールドにエイリアスを追加できます：

SELECT
    a.id,
    a.name,
    a.category_id,
    b.id AS catId,
    b.name AS catName
FROM
    product AS a
LEFT JOIN
    category AS b ON a.category_id = b.category.id