product_id|soldOut|stock
------------------
1         |Yes    |0
2         |Yes    |0
3         |No     |NULL
4         |No     |1

SELECT p.product_id, IF(pv.stock > 0 OR pv.stock IS NULL, 'No', 'Yes') as SoldOUt
FROM products p
JOIN product_variants pv ON (`pv`.`product_id` = `p`.`product_id`)
GROUP BY p.product_id