mysql number of rows per hour

Question

I have a table where each row has a timestamp. Is there a query I can run that can count the number of rows per hour. Outputting a row for each hour with a field for the number of rows encompassed?

e.g

id timestamp

10 2010-09-19 21:05:05

11 2010-09-19 22:05:30

12 2010-09-19 23:05:05

13 2010-09-19 23:05:05

number of rows | hour
1                21
1                22
2                23

I guess there might be problems with using the hour as there will be duplicate hours... so maybe '2010-09-19 21' or just a number representing the period (e.g in the example 2010-09-19 21 is 1)

TIA

Answer 1

It depends on how big your date range is. If all dates fall within a month, e.g., you could do this:

select day(timestamp) as Day, hour(timestamp) as Hour, count(*) as Count
from MyTable
where timestamp between :date1 and :date2
group by day(timestamp), hour(timestamp)

You can group by year and month as well if you need to further separate your data.

Answer 2

SELECT HOUR(hour) as hour, COUNT(*) as num_rows FROM `TABLE` GROUP BY HOUR(hour)