不能有条件地为列指定列名。假设得到三行结果。在每一行中，计算出不同的iif结果，因此其含义可能不同。那么应该如何调用包含这些值的列呢？
可以包括所有三个附加列。或者可以创建一个“generic”列来保存“calculation”结果。
无论哪种方式，您都需要一个额外的列来指示结果类型（基价、货币折扣、百分比折扣或单价）。
为了得到一些启发，我在这里加入了我对你的质疑的变体。它包括上述所有策略。注意，我首先转换了 IIF s到a CASE -阻止。我试着尽可能小心，但是我没有测试这个查询，所以它可能包含bug。

SELECT
  product.productName,
  price.basePrice,
  price.promotionValid,
  discount.finalPrice,
  -- separate calculation result fields:
  discount.finalPrice - price.basePrice AS currencyDiscount,
  (price.basePrice / discount.finalPrice) * 100 AS percentageDiscount,
  discount.finalPrice / discount.bundleQuantity AS unitPrice,
  -- single column containing the conditional calculation result:
  CASE
      WHEN DATEDIFF(dd, startPromotion, GETDATE()) >= 0 AND DATEDIFF(dd, GETDATE(), endPromotion) >= 0 THEN price.basePrice
      WHEN price.promotionValid = 0 THEN price.basePrice
      WHEN discount.promotionType = 0 AND discount.discountMethod = 0 THEN discount.finalPrice - price.basePrice --currencyDiscount
      WHEN discount.promotionType = 0 AND discount.discountMethod = 1 THEN (price.basePrice / discount.finalPrice) * 100 --percentageDiscount
      WHEN discount.promotionType = 1 THEN discount.finalPrice / discount.bundleQuantity --unitPrice
      ELSE price.basePrice
  END AS calculationResult,
  -- column indicating the effective result type:
  CASE
      WHEN DATEDIFF(dd, startPromotion, GETDATE()) >= 0 AND DATEDIFF(dd, GETDATE(), endPromotion) >= 0 THEN 'base price (promotion not active)'
      WHEN price.promotionValid = 0 THEN 'base price (promotion not valid)'
      WHEN discount.promotionType = 0 AND discount.discountMethod = 0 THEN 'currency discount'
      WHEN discount.promotionType = 0 AND discount.discountMethod = 1 THEN 'percentage discount'
      WHEN discount.promotionType = 1 THEN 'unit price'
      ELSE 'base price (no promotion found)'
  END AS calculationResultType
FROM
  product
  INNER JOIN price ON price.productName = product.productName
  INNER JOIN discount ON price.productName = discount.productName