MySQL Simple and Clean Solution
Please Upvote if it helped you !!!\n\nselect product_id ,\'store1\' as store ,store1 as price from products where store1 is not null\nunion \nselect product_id 
**Please Upvote if it helped you !!!**\n```\nselect product_id ,\'store1\' as store ,store1 as price from products where store1 is not null\nunion \nselect product_id ,\'store2\' as store ,store2 as price from products where store2 is not null\nunion \nselect product_id ,\'store3\' as store,store3 as price from products where store3 is not null\norder by product_id;\n```
['Union Find', 'MySQL']