在多对多关系上创建简单选择

Question

让我说我有这个表：

country:
country_id,
country_name;

currency:
currency_id,
currency_name;

country_currency:
country_id,
currency_id;

我想选择并显示所有拥有两种或更多货币的国家/地区。我的查询应该如何？

Answer 1

使用公共列加入所有表，然后按country_id对结果进行分组，并计算每个组的行数（或货币），如下所示。

SELECT A.COUNTRY_ID, COUNT(*) NUMBER_OF_CURRENCY 
FROM COUNTRY A 
    INNER JOIN COUNTRY_CURRENCY B ON A.COUNTRY_ID = B.COUNTRY_ID 
    INNER JOIN CURRENCY C ON B.CURRENCY_ID=C.CURRENCY_ID
GROUP BY A.COUNTRY_ID 
HAVING NUMBER_OF_CURRENCY > 1 ;

更好的查询是：

SELECT COUNTRY_NAME 
FROM COUNTRY 
WHERE COUNTRY_ID IN (
                    SELECT COUNTRY_ID 
                    FROM COUNTRY_CURRENCY 
                    GROUP BY COUNTRY_ID 
                    HAVING COUNT(*) > 1
                     );