[LeetCode] 1795. Rearrange Products Table

😎풀이

1. store1 이 NULL이 아니라면, store1 점수 출력
2. store2 이 NULL이 아니라면, store2 점수 출력
3. store3 이 NULL이 아니라면, store3 점수 출력
4. 해당 컬럼을 UNION으로 엮어 같이 조회회

SELECT 
    p1.product_id AS product_id, 
    'store1' AS store,
    p1.store1 AS price
FROM Products AS p1 
WHERE p1.store1 IS NOT NULL
UNION
SELECT 
    p2.product_id AS product_id, 
    'store2' AS store,
    p2.store2 AS p2
FROM Products AS p2 
WHERE p2.store2 IS NOT NULL
UNION
SELECT 
    p3.product_id AS product_id, 
    'store3' AS store,
    p3.store3 AS price
FROM Products AS p3 
WHERE p3.store3 IS NOT NULL