MySQL ORDER BY，GROUP BY内JOIN

Question

我一直在寻找一段时间，并没有找到一个有效的例子。希望你能发现明显的错误！

SELECT 
     Timestamp 
     , i_currency.Code AS Code 
     , Conversion 
    FROM 
     i_convert(
      SELECT 
       Timestamp 
       , Conversion 
      FROM 
       i_convert 
      ORDER BY Timestamp DESC 
     ) 
     JOIN i_currency 
      ON i_convert.CurrencyID = i_currency.CurrencyID 
    GROUP BY Code 

我不确定JOIN应该在哪里，它应该在括号里，还是在外面或两者都有？我已经尝试了所有三个没有运气，不断得到：

You have an error... ...near '(SELECT Timestamp , Conversion FROM i_convert ORDER ' 

Answer 1

最初的查询没有意义。它使用GROUP BY，但试图选择既不分组列也不聚合函数的列。对于任何给定的结果行，预计哪种时间戳和哪种转换？

我想目标是选择每种货币的最新转换。这可能看起来更像这样：

SELECT 
    latest.Timestamp AS Timestamp 
    , i_currency.Code AS Code 
    , i_convert.Conversion AS Conversion 
FROM i_currency 
    JOIN i_convert 
     ON i_convert.CurrencyID = i_currency.CurrencyID 
    JOIN (
     SELECT MAX(Timestamp) as Timestamp, CurrencyId 
     FROM i_convert 
     GROUP BY CurrencyId 
    ) latest 
     ON i_convert.Timestamp = latest.Timestamp 
      AND i_convert.CurrencyId = latest.CurrencyId