Table: `Products`

    +-------------+---------+
    | Column Name | Type    |
    +-------------+---------+
    | product_id  | int     |
    | store1      | int     |
    | store2      | int     |
    | store3      | int     |
    +-------------+---------+
    product_id is the primary key for this table.
    Each row in this table indicates the product's price in 3 different stores: store1, store2, and store3.
    If the product is not available in a store, the price will be null in that store's column.

Write an SQL query to rearrange the `Products` table so that each row has `(product_id, store, price)`. If a product is not available in a store, do not include a row with that `product_id` and `store` combination in the result table.

Return the result table in any order.

The query result format is in the following example.

Example 1:

    Input:
    Products table:
    +------------+--------+--------+--------+
    | product_id | store1 | store2 | store3 |
    +------------+--------+--------+--------+
    | 0          | 95     | 100    | 105    |
    | 1          | 70     | null   | 80     |
    +------------+--------+--------+--------+
    Output:
    +------------+--------+-------+
    | product_id | store  | price |
    +------------+--------+-------+
    | 0          | store1 | 95    |
    | 0          | store2 | 100   |
    | 0          | store3 | 105   |
    | 1          | store1 | 70    |
    | 1          | store3 | 80    |
    +------------+--------+-------+
    Explanation:
    Product 0 is available in all three stores with prices 95, 100, and 105 respectively.
    Product 1 is available in store1 with price 70 and store3 with price 80. The product is not available in store2.