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Problem

Table: Products

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| product_id  | int     |
| store_name1 | int     |
| store_name2 | int     |
|      :      | int     |
|      :      | int     |
|      :      | int     |
| store_namen | int     |
+-------------+---------+
product_id is the primary key for this table.
Each row in this table indicates the product's price in n different stores.
If the product is not available in a store, the price will be null in that store's column.
The names of the stores may change from one testcase to another. There will be at least 1 store and at most 30 stores.

Important note: This problem targets those who have a good experience with SQL. If you are a beginner, we recommend that you skip it for now.

Implement the procedure UnpivotProducts to reorganize the Products table so that each row has the id of one product, the name of a store where it is sold, and its price in that store. 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. There should be three columns: product_id, store, and price.

The procedure should return the table after reorganizing it.

Return the result table in any order.

The query result format is in the following example.

Example 1:

Input: 
Products table:
+------------+----------+--------+------+------+
| product_id | LC_Store | Nozama | Shop | Souq |
+------------+----------+--------+------+------+
| 1          | 100      | null   | 110  | null |
| 2          | null     | 200    | null | 190  |
| 3          | null     | null   | 1000 | 1900 |
+------------+----------+--------+------+------+
Output: 
+------------+----------+-------+
| product_id | store    | price |
+------------+----------+-------+
| 1          | LC_Store | 100   |
| 1          | Shop     | 110   |
| 2          | Nozama   | 200   |
| 2          | Souq     | 190   |
| 3          | Shop     | 1000  |
| 3          | Souq     | 1900  |
+------------+----------+-------+
Explanation: 
Product 1 is sold in LC_Store and Shop with prices of 100 and 110 respectively.
Product 2 is sold in Nozama and Souq with prices of 200 and 190.
Product 3 is sold in Shop and Souq with prices of 1000 and 1900.

Code

CREATE PROCEDURE UnpivotProducts()
BEGIN
	# Write your MySQL query statement below.
    set session group_concat_max_len = 1000000;
    
    set @macro = null;
    
	select group_concat(
        concat(
            'select product_id, "', `column_name`, '" as store, ', `column_name`,
            ' as price from products where ', `column_name`, ' is not null'
        ) separator ' union '
    )
        into @macro
        from `information_schema`.`columns`
        where `table_schema`='test' and `table_name`='products' and `column_name` != 'product_id';
    
    prepare sql_query from @macro;
    execute sql_query;
    deallocate prepare sql_query;
END