SQL Server：仅选择嵌套表中存在的项目答案

【问题标题】：SQL Server: select only items that exist in nested tableSQL Server：仅选择嵌套表中存在的项目
【发布时间】：2014-07-18 19:01:10
【问题描述】：

我正在使用以下查询从一个表中选择货币列表，并从另一个表中获取相应的汇率，该表目前运行良好。

这里有人可以告诉我如何更改它，以便它只选择两个表中都存在的货币，即在表 E (C.currencyCode = E.from_currency) 中具有可用汇率的货币？

SELECT      C.currency,
            C.currencyCode,
            (
                SELECT      E.exchange_rate AS exchangeRate
                FROM        Exchange_Rates E
                WHERE       E.from_currency = C.currencyCode
                AND         E.to_currency = 'USD'
                FOR XML     PATH(''), ELEMENTS, TYPE
            )
FROM        Currencies C
ORDER BY    C.sortID, C.currency
FOR XML PATH('currencies'), ELEMENTS, TYPE, ROOT('ranks')

非常感谢您提供的任何帮助，蒂姆。

【问题讨论】：

标签： sql sql-server select stored-procedures nested-queries

【解决方案1】：

你可以试试这个吗？

SELECT      C.currency,
            C.currencyCode,
            (
                SELECT      E.exchange_rate AS exchangeRate
                FROM        Exchange_Rates E
                WHERE       E.from_currency = C.currencyCode
                AND         E.to_currency = 'USD'
                FOR XML     PATH(''), ELEMENTS, TYPE
            )
FROM        Currencies C
        JOIN Exchange_Rates E ON C.currencyCode = E.from_currency
ORDER BY    C.sortID, C.currency
FOR XML PATH('currencies'), ELEMENTS, TYPE, ROOT('ranks')

【讨论】：

非常感谢 - 让我检查一下。
这很好用 - 感谢您的快速帮助！我会尽快接受。 :)

【解决方案2】：

SELECT      C.currency,
            C.currencyCode,
            (
                SELECT      E.exchange_rate AS exchangeRate
                FROM        Exchange_Rates E
                WHERE       E.from_currency = C.currencyCode
                AND         E.to_currency = 'USD'
                FOR XML     PATH(''), ELEMENTS, TYPE
            )
FROM        Currencies C
        INNER JOIN (Select MIN(exchange_rate),
        from_currency 
            from Exchange_Rates
            GROUP BY from_currency )E ON C.currencyCode = E.from_currency
ORDER BY    C.sortID, C.currency
FOR XML PATH('currencies'), ELEMENTS, TYPE, ROOT('ranks')

【讨论】：