在多对多关系上创建简单的选择

Question

假设我有这张桌子：

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多种货币的国家。我的查询应该是什么样子？

Answer 1

将所有表格与公共列连接起来，然后按 country_id 对结果进行分组，并计算每组的行数（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
                     );