select distinct year,month,day,
round((avg((tend - tstart)/60)),2) Duration,
count(*) INTO OUTFILE 'c:/aavso/ngdc_data/tdur_yr_id.csv' FIELDS TERMINATED BY ','
from ngdcsid, aavso_id 
where year between '1958' and '2008'
and substr(station,1,4) = substr(ID,1,4)
group by year,month,day
having (count(*) <= 50 and round((avg((tend - tstart)/60)),2) >= 0)
order by year,month,day;