SQL Server:come selezionare tutti i giorni in un intervallo di date anche se non esistono dati per alcuni giorni

Puoi utilizzare un CTE ricorsivo per creare il tuo elenco di 30 giorni, quindi unirlo ai tuoi dati

--test
select cast('05 jan 2011' as datetime) as DT, 1 as val into #t
union all select CAST('05 jan 2011' as datetime), 1 
union all select CAST('29 jan 2011' as datetime), 1 

declare @start datetime = '01 jan 2011'
declare @end   datetime = dateadd(day, 29, @start)

;with amonth(day) as
(
    select @start as day
        union all
    select day + 1
        from amonth
        where day < @end
)
select amonth.day, count(val)
    from amonth 
    left join #t on #t.DT = amonth.day
group by amonth.day


>>

2011-01-04 00:00:00.000 0
2011-01-05 00:00:00.000 2
2011-01-06 00:00:00.000 0
2011-01-07 00:00:00.000 0
2011-01-08 00:00:00.000 0
2011-01-09 00:00:00.000 0
...