GROUP BY用法
聚合函数只能产生1个汇总数据,group by 用于先分组,对每个分组产生1个汇总数据。
group by 必然是和聚合函数一起使用的。
Having 用于过滤某些分组,必须跟着group by 使用。
例子:
SET NOCOUNT ON
DECLARE @StartDate DateTime
DECLARE @EndDate DateTime
SET @StartDate = DateAdd(mm,-1,GetDate())
SET @EndDate = GetDate()
SET NOCOUNT OFF
SELECT TagName,max(Value)
FROM History
WHERE TagName like 'CLFJJK1_00%JI802'
AND wwRetrievalMode = 'Cyclic'
AND wwCycleCount = 3
AND wwVersion = 'Latest'
AND DateTime >= @StartDate
AND DateTime <= @EndDate
group by TagName