beats 99.89% with understandable code
\n##for store 1 \nwith t1 as (select product_id, \'store1\' as store, store1 as price from Products where store1 is not null),\n\n##for store 2\nt2 as (select p
```\n##for store 1 \nwith t1 as (select product_id, \'store1\' as store, store1 as price from Products where store1 is not null),\n\n##for store 2\nt2 as (select product_id, \'store2\' as store, store2 as price from Products where store2 is not null),\n\n##for store 3\nt3 as (select product_id, \'store3\' as store, store3 as price from Products where store3 is not null)\n\n\nselect * from  (select * from t1 x UNION select * from t2 y UNION select * from t3 z) as X;\n```\n\nplease upvote if this is helpful.
['MySQL']