1795 Rearrange Products Table.md

1795. Rearrange Products Table

https://leetcode.cn/problems/rearrange-products-table/
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.

union

# Write your MySQL query statement below
SELECT
    product_id,
    "store1" AS store,
    store1 AS price
FROM Products
WHERE store1 IS NOT NULL
UNION
SELECT
    product_id,
    "store2" AS store,
    store2 AS price
FROM Products
WHERE store2 IS NOT NULL
UNION
SELECT
    product_id,
    "store3" AS store,
    store3 AS price
FROM Products
WHERE store3 IS NOT NULL