Question

我总共3 tables。我试图选择成本食品的价格必须是最高的，类别名称必须属于有机食品。

我提出了以下SQL QUERY，但我收到了无效的关系错误。

有人可以指出问题可能是什么吗？

由于

SELECT PRICE, LABEL
FROM FOOD
WHERE FOOD.FOOD_ID = (
    SELECT FOOD_ID
    FROM FOOD_CATEGORY
    WHERE FOOD_CATEGORY.FOOD_ID = (
       SELECT FOOD_ID
       FROM CATEGORY
       WHERE NAME = 'ORGANIC'
    )
);

食物

FOOD_ID
LABEL
PRICE

FOOD_CATEGORY

FOOD_ID
CATEGORY_ID

CATEGORY

CATEGORY_ID
NAME

关系

FOOD.FOOD_ID = FOOD_CATEGORY.FOODID 

FOOD_CATEGORY.CATEGORYID = CATEGORY.CATEGORYID

Answer 1

我怀疑你有更多的食物属于“有机”类别。否则就不需要表FOOD_CATEGORY。表FOOD_CATEGORY用于创建多对多关系。

为什么不呢：

SELECT PRICE, LABEL
FROM FOOD
JOIN FOOD_CATEGORY ON (FOOD_CATEGORY.FOOD_ID = FOOD.FOOD_ID)
JOIN CATEGORY ON (CATEGORY.CATEGORY_ID = FOOD_CATEGORY.CATEGORY_ID)
WHERE CATEGORY.NAME = 'ORGANIC';

或使用IN而不是=

SELECT PRICE, LABEL
FROM FOOD
WHERE FOOD.FOOD_ID IN (
    SELECT FOOD_ID
    FROM FOOD_CATEGORY
    WHERE FOOD_CATEGORY.CATEGORY_ID = (
       SELECT CATEGORY_ID
       FROM CATEGORY
       WHERE NAME = 'ORGANIC'
    )
);

Answer 2

因为CATEGORY_ID代替FOOD_ID

SELECT PRICE, LABEL
FROM FOOD
WHERE FOOD.FOOD_ID = (
    SELECT FOOD_ID
    FROM FOOD_CATEGORY
    WHERE FOOD_CATEGORY.FOOD_ID = (
       SELECT CATEGORY_ID
       FROM CATEGORY
       WHERE NAME = 'ORGANIC'
    )
);

具有无效关系错误的SQL嵌套查询（ORA-00920错误）

2 个答案: