使用Case语句时MIN和MAX的问题

Question

我正在尝试使用各种聚合函数生成摘要报告：MIN，MAX，SUM等。我遇到的问题是当我在使用case语句时尝试获取字段的MIN和MAX时。当我使用case语句时，我无法获得字段的MIN值。我可以用示例数据和sql语句来解释它：

字段：AccountNumber，Symbol，TradeDate，TransactionType，Price，Quantity，Amount表：交易

AccountNumber, Symbol, TradeDate, TransactionType, Price, Quantity, Amount
123,"XYZ",1/2/2011,"Buy",15,100,1500
123,"XYZ",1/2/2011,"Buy",10,50,500
123,"XYZ",1/2/2011,"Sell",20,100,2000
456,"ABC",1/3/2011,"Buy",10,20,200
456,"ABC",1/3/2011,"Buy",15,30,450
789,"DEF",1/4/2011,"Sell",30,100,3000

查询：

SELECT AccountNumber,
       Symbol,
       SUM(case when TransactionType = "Buy" then 1 else 0) as TotalBuys,
       SUM(case when TransactionType = "Sell" then 1 else 0) as TotalSells,
       MIN(case when TransactionType = "Buy" then Price else 0) as MinBuy,
       MAX(case when TransactionType = "Buy" then Price else 0) as MaxBuy,
       MIN(case when TransactionType = "Sell" then Price else 0) as MinSell,
       MAX(case when TransactionType = "Sell" then Price else 0) as MaxSell,
       MIN(Price) as MinPrice,
       MAX(Price) as MaxPrice
  FROM Trades
Group By AccountNumber, Symbol

我期待的是以下结果：

AccountNumber, Symbol, TotalBuys, TotalSells, MinBuy, MaxBuy, MinSell, MaxSell, MinPrice, MaxPrice
123,"XYZ",2,1,10,15,20,20,10,20
456,"ABC",2,0,10,15,0,0,10,15
789,"DEF",0,1,0,0,30,30,30,30

但是，我得到以下结果：

AccountNumber, Symbol, TotalBuys, TotalSells, MinBuy, MaxBuy, MinSell, MaxSell, MinPrice, MaxPrice
123,"XYZ",2,1,**0**,15,**0**,20,**0**,20
456,"ABC",2,0,10,15,0,0,10,15
789,"DEF",0,1,0,0,30,30,30,30

当每个分组有两个不同的TransactionType时，Min字段（MinBuy，MinSell和MinPrice）将显示为0而不是预期。我在sql语句上做错了什么？有没有其他方法可以获得理想的结果？

Answer 1

最小值介于0和正数之间为0，您应该更改：

MIN(case when TransactionType = "Buy" then Price else 0)

通过

MIN(case when TransactionType = "Buy" then Price else Null)

Null不在聚合函数中计算。

就这样。

6年后编辑：

正如P5Coder所说，没有else条款就足够了，我想end在一些数据库品牌上是强制性的。这里是：

MIN(case when TransactionType = "Buy" then Price end)