您应该像这样通过联接进行更新

You should just being doing the update across a join like this

UPDATE
t1 INNER JOIN t2 ON t1.variant_id = t2.variant_id
SET t1.product_code = t2.value
WHERE t2.key_id = 10
AND t2.value IS NOT NULL

在这种情况下，无需担心空值，因为内部联接只会选择两个表中都存在variant_id的行.

There is no need to worry about nulls in that case as the inner join will only select rows where the variant_id exists in both tables.