如何将每次评估的CASE语句限制为1行

Question

所以我有一个包含2行的简单查询：

SELECT DISTINCT
    o.OrderNo,
    CASE
        WHEN od.PartNo LIKE '%Amend%' AND od.User_Currency2 IS NOT NULL THEN od.User_Currency2
        WHEN od.PartNo LIKE '%Amend%' AND od.User_Currency1 IS NOT NULL THEN od.User_Currency1
        ELSE o.OrderTotal
        END AS [OrderTotal] 
FROM OrderDet od
   JOIN Orders o ON od.OrderNo = o.OrderNo
WHERE o.OrderNo LIKE '%E'
ORDER BY o.OrderNo

在我的CASE语句中，对于每个订单，如果满足第一个条件，我希望它只返回那个并忽略其他2个选项，条件＃2相同，如果前两个条件失败，则默认为ELSE

问题是我每个订单都有多行，如下所示：

如何限制每个订单只有1条记录？我认为CASE语句按顺序进行评估并且只会吐出1个值，但在我的情况下，每个订单都会从每个订单的CASE语句中返回多个结果

Answer 1

使用一系列查询联合并按User_Currency2 / User_Currency1值过滤，如下所示：

SELECT DISTINCT
    o.OrderNo,
    od.User_Currency2 AS [OrderTotal]
FROM OrderDet od
   JOIN Orders o ON od.OrderNo = o.OrderNo
WHERE
  o.OrderNo LIKE '%E' AND
  od.PartNo LIKE '%Amend%' AND
  od.User_Currency2 IS NOT NULL
UNION
SELECT DISTINCT
    o.OrderNo,
    od.User_Currency2 AS [OrderTotal]
FROM OrderDet od
   JOIN Orders o ON od.OrderNo = o.OrderNo
WHERE
  o.OrderNo LIKE '%E' AND
  od.PartNo LIKE '%Amend%' AND
  od.User_Currency2 IS NULL AND
  od.User_Currency1 IS NOT NULL
UNION
SELECT DISTINCT
    o.OrderNo,
    od.OrderTotal
FROM OrderDet od
   JOIN Orders o ON od.OrderNo = o.OrderNo
WHERE
  o.OrderNo LIKE '%E' AND
  od.PartNo LIKE '%Amend%' AND
  od.User_Currency2 IS NULL AND
  od.User_Currency1 IS NULL
ORDER BY
  o.OrderNo