您可以在子查询中使用ROW_NUMBER()函数来获取客户的第一个和第二个交易日期，然后对该子查询的结果使用条件MAX窗口函数。

SELECT CustomerID, DATEPART(week,CustTrans) AS Trans_week,
       DATEPART(week, MAX(CASE rn WHEN 1 THEN CustTrans END) OVER (PARTITION BY CustomerID)) first,
       DATEPART(week, MAX(CASE rn WHEN 2 THEN CustTrans END) OVER (PARTITION BY CustomerID)) second
FROM
  (
    SELECT *,
      ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY CustTrans) rn
    FROM trydata
  ) T
ORDER BY CustomerID, Trans_week

请参阅SQL Server上的demo。
根据您在注解中的请求，如果您希望为每个客户仅选择一行来显示第一周和第二周，请使用以下查询：

SELECT CustomerID,
   DATEPART(week, MAX(CASE rn WHEN 1 THEN CustTrans END)) first,
   DATEPART(week, MAX(CASE rn WHEN 2 THEN CustTrans END)) second
FROM
  (
    SELECT *,
      ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY CustTrans) rn
    FROM trydata
  ) T
WHERE rn <= 2
GROUP BY CustomerID
ORDER BY CustomerID

请参见demo。

with cte (RN,CustomerID, FirstWeek,SecondWeek ) as
( SELECT ROW_NUMBER() over(partition by CustomerID  ORDER BY CustomerID  )  RN, CustomerID,FirstWeek, isnull((select TOP 1 (DATEPART(week,CustTrans))   
                from trydata c 
                    where c.CustomerID = SRC.CustomerID AND DATEPART(week,C.CustTrans) > SRC.FirstWeek  
                    ORDER BY DATEPART(week,C.CustTrans) ),'0')  AS SecondWeek 
FROM (  
    SELECT CustomerID,DATEPART(week,CustTrans) TransWeek,
      (select MIN(DATEPART(week,CustTrans)) from trydata c where c.CustomerID = trydata.CustomerID) AS FirstWeek
    FROM trydata
) SRC )

select CustomerID,FirstWeek,SecondWeek from cte where RN = 1

输出量：
例二：

WITH CTE (CustomerID,FIrstWeek,RN) AS (
    SELECT CustomerID,MIN(DATEPART(week,CustTrans)) TransWeek,
    ROW_NUMBER() over(partition by CustomerID  ORDER BY DATEPART(week,CustTrans) asc ) FROM TryData 
    GROUP BY CustomerID,DATEPART(week,CustTrans)
) 

SELECT CTE.CustomerID, CTE.FIrstWeek,  
     (select TOP 1 (DATEPART(week,c.CustTrans))   
        from trydata c 
            where c.CustomerID = CTE.CustomerID AND DATEPART(week,C.CustTrans) > CTE.FIrstWeek 
                )   SecondWeek 
FROM CTE  
WHERE RN = 1

FIddle Demo
编辑：这可以用更简单的方法和更简单的方法来完成。