Question

For each month of the year in 2012, output the number of days in that month. Format the output as an integer column containing the month of the year, and a second column containing an interval data type.
Schema reminder
DB schema

Expected Results

month length
1 31 days
2 29 days
3 31 days
4 30 days
5 31 days
6 30 days
7 31 days
8 31 days
9 30 days
10 31 days
11 30 days
12 31 days

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Answers and Discussion Show

select 	extract(month from cal.month) as month,
	(cal.month + interval '1 month') - cal.month as length
	from
	(
		select generate_series(timestamp '2012-01-01', timestamp '2012-12-01', interval '1 month') as month
	) cal
order by month;          

This answer shows several of the concepts we've learned. We use the GENERATE_SERIES function to produce a year's worth of timestamps, incrementing a month at a time. We then use the EXTRACT function to get the month number. Finally, we subtract each timestamp + 1 month from itself.

It's worth noting that subtracting two timestamps will always produce an interval in terms of days (or portions of a day). You won't just get an answer in terms of months or years, because the length of those time periods is variable.

Subtracting two timestamps will give you the interval you're looking for. You can use the EXTRACT function to get the month from a timestamp.

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