您可以使用子查询为每个客户的每个销售分配行号，按升序日期排序，然后仅选择第一行：

SELECT "Customer ID", "Date", "Amount"
FROM (
  SELECT "Customer ID", "Date", "Amount",
         ROW_NUMBER() OVER (PARTITION BY "Customer ID" ORDER BY "Date") AS rn
  FROM Sales) s
WHERE rn = 1

sqlfiddle演示

尼克的解决方案可能是最有效的，但如果你想使用 GROUP BY 在这里，你可以这样做：

SELECT
    c.ID,
    c.Name,
    s1.Date,
    s1.Amount
FROM Customer c
INNER JOIN Sales s1 ON c.ID = s1."Customer ID"
INNER JOIN
(
    SELECT "Customer ID", MIN(Date) AS FirstSaleDate
    FROM Sales
    GROUP BY "Customer ID"
) s2
    ON s1."Customer ID" = s2."Customer ID" AND
       s1.Date = s2.FirstSaleDate
ORDER BY
    c.ID,
    c.Name;

在oracle中，可以使用 keep :

select customer_id, min(date) as first_sales_date,
       max(amount) keep (dense_rank first order by date asc) as first_amount
from sales
group by customer_id;