how do I group by by hour in postgresql with a “time” field?

Question

I have a table with a column ctime of type time without time zone.

  |   cdate    |  ctime   
  +------------+----------
  | 2016-12-2         


        

Answer 1

use date_trunc:

group by cdate, date_trunc('hour', ctime)

Answer 2

I think date_part('hour', ctime) is a better choice if you just want to group based on the hour.

date_part('hour', timestamp '2001-02-16 20:38:40')    ---> 20
date_trunc('hour', interval '2 days 3 hours 40 minutes')    ---> 2 days 03:00:00