为什么在尝试执行此 SQL 语句时会收到 Ambiguous Column name CountryID 错误消息：

Question

SELECT
    CD.CountryID, CD.GrossLimit, CD.UnsecuredLimit,
    SUM(LT1.Amount), SUM(LT1.Unsecured), (100*SUM(LT1.Unsecured) / CD.UnsecuredLimit) as PercOverCountryLimit
   FROM CountryDetail CD 
   INNER JOIN
    (
    SELECT CompanyName AS Company, CollateralSName as Collateral, SUM(Amount) AS Amount,  
           SUM(Usecured) AS Unsecured, LT.Date as Date, Max(LT.CountryID) as CountryID
        FROM Loanstotal LT
        WHERE YearMonth = @YearMonth
        GROUP BY CompanyName, CollateralSName, LT.Date
    ) LT1
   ON CD.CountryID = LT1.CountryID 
   GROUP BY CountryID, GrossLimit, UnsecuredLimit

Answer 1

Because you're grouping by CountryID which is being returned by both tables: CountryDetail and your sub-query. Change your GROUP BY to GROUP BY CD.CountryID, GrossLimit, UnsecuredLimit .

In fact, I think as a best practice you should change all your column references so they're qualified with the proper aliases:

GROUP BY CD.CountryID, CD.GrossLimit, CD.UnsecuredLimit