declare @date datetime
set @date = '...'
Update userinfo set date1 = (case when datediff(day,date1,@date) * datediff(day,date2,@date) <0 then @date else date1 end)
set @date = '...'
Update userinfo set date1 = (case when datediff(day,date1,@date) * datediff(day,date2,@date) <0 then @date else date1 end)
declare @date datetime
set @date = '...'
Update userinfo set date1 = (case when (date1>@date and date2<@date) or (date1<@date and date2>@date) then @date else date1 end)