String sql="select id,timedata,sum(ball) as total from buy where timedata between '"+date1+"' and '"+date2+"' group by timedata,id";
//ball参与聚合,不用出现在group by中

------解决方案--------------------
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C# code

String sql="select id,timedata,sum(ball) as total from buy where timedata between '"+date1+"' and '"+date2+"' group by timedata,id";
//ball参与聚合,不用出现在group by中


------解决方案--------------------

select convert(varchar(10),timedata,120) date,sum(ball) as total
from tb
where timedata between ... and ...
group by convert(varchar(10),timedata,120)