Question

How do I get a maximium daily value of a numerical field over a year in MS-SQL

Answer 1

This would query the daily maximum of value over 2008:

select 
    datepart(dayofyear,datecolumn)
,   max(value)
from yourtable
where '2008-01-01' <= datecolumn and datecolumn < '2009-01-01'
group by datepart(dayofyear,datecolumn)

Or the daily maximum over each year:

select 
    datepart(year,datecolumn), 
,   datepart(dayofyear,datecolumn)
,   max(value)
from yourtable
group by datepart(year,datecolumn), datepart(dayofyear,datecolumn)

Or the day(s) with the highest value in a year:

select 
    Year = datepart(year,datecolumn),
,   DayOfYear = datepart(dayofyear,datecolumn)
,   MaxValue = max(MaxValue)
from yourtable
inner join (
    select 
        Year = datepart(year,datecolumn), 
    ,   MaxValue = max(value)
    from yourtable
    group by datepart(year,datecolumn)
) sub on 
    sub.Year = yourtable.datepart(year,datecolumn)
    and sub.MaxValue = yourtable.value
group by 
    datepart(year,datecolumn),
    datepart(dayofyear,datecolumn)