How to GROUP BY date with a timestamp field in Postgres?

Question

Lets say that I have 3 rows of data:

id  product_uuid                version_uuid                created_at
22  586d8e21b9529d14801b91bd    5a711a0094df04e23833d8ef    2018-02-10 19:51:15.075-05
23  586d8e21b9529d14801b91bd    5a711a0094df04e23833d8ef    2018-02-10 19:51:16.077-07
24  586d8e21b9529d14801b91bd    5a711a0094df04e23833d8ef    2018-02-11 19:51:15.077-05

And I want to group them by day via the created_at column.

SELECT created_at::date, COUNT(*)
FROM table_name
WHERE product_uuid = '586d8e21b9529d14801b91bd'
AND created_at > now() - interval '30 days'
GROUP BY created_at
ORDER BY created_at ASC

I would expect this to yield 2 rows:

created_at   count
2018-02-10   2
2018-02-11   1

But I actually get 3 rows:

created_at   count
2018-02-10   1
2018-02-10   1
2018-02-11   1

I realize that GROUP BY is still grouping by the fine-grain timestamp, but I'm not sure how to make Postgres use the truncated date instead.

Answer 1

You need to truncate in the GROUP BY as well:

SELECT created_at::date, COUNT(*)
FROM table_name
WHERE product_uuid = '586d8e21b9529d14801b91bd' AND
      created_at > now() - interval '30 days'
GROUP BY created_at::date
ORDER BY created_at::date ASC;

Your version is aggregating by each date/time value but only showing the date component.

Also, I would recommend that you use current_date rather than now() so the first date is not truncated.