專案中經常碰到需要在資料錶行中取最大值、最小值、平均值問題，SQL資料庫不支援此功能，所以我們需要轉換一下。比如說有以下表結構：

圖1資料庫表結構

我們需要選取出每個時間點（DateTime）每個裝置（PositionID）的所有數值（Value1，Value2，Value3，Value4）平均值、最大值、最小值，可採用如下方法：

select t.[datetime] as [DateTime], t.positionid as Positionid, 
max(t.value1) as MaxValue, min(t.value1) as MinValue, avg(t.value1) as AvgValue
from
(select [datetime], positionid, value1 from historydata union
select [datetime], positionid, value2 from historydata union
select [datetime], positionid, value3 from historydata union
select [datetime], positionid, value4 from historydata) t
where positionid in 
(
select positionid
from 表
)
group by t.[datetime], [positionid]
order by positionid, [datetime]