Count of Daily Users in PostgreSQL - postgresql

Question

I have a database that looks like this
users
phone created
8001234578 1540231160
9001234578 1540220360
1001234578 1540144760
2001234578 1540058360
Note that the created column is a unix timestamp.
I want to group them by users created on the same day with a count of users, so the above example database should return something like this.
[
{day: '10/22/2018', count: 2},
{day: '10/21/2018', count: 1},
{day: '10/20/2018', count: 1},
]
I tried learning about the to_char command but I couldn't figure it out, this is what I tried:
SELECT
to_char(created, 'Day') as day,
COUNT(*) as count
FROM users
WHERE created >= ${startDate} AND created <= ${endDate}
GROUP BY to_char(created, 'Day')
It returned this:
[{day: ' .ay', count: '4'}]

Answer 1

First, if it's within your control, you should consider using Postgres's built in TIMESTAMP data type, instead of storing your dates as UNIX timestamps. This will make your queries much easier.
That being said, if you're stuck with using UNIX timestamps (presumably stored as integers), you'll have to convert them to a TIMESTAMP anyways to get what you want. You can use the TO_TIMESTAMP to convert from the UNIX timestamp, and use the DATE_TRUNC function to just get the date portion:
SELECT
DATE_TRUNC('day', TO_TIMESTAMP(created)),
COUNT(*)
FROM users
GROUP BY DATE_TRUNC('day', TO_TIMESTAMP(created))
Of course, if you're storing the dates as a Postgres TIMESTAMP, it's the same query, but simpler:
SELECT
DATE_TRUNC('day', created),
COUNT(*)
FROM users
GROUP BY DATE_TRUNC('day', created)

Answer 2

SELECT date_trunc('day', users.created) "day", count(*) as count
FROM users
WHERE created >= ${startDate} AND created <= ${endDate}
GROUP BY 1

Answer 3

You can set datetime format, so you got result like this
postgres# select to_char(now(), 'DD Mon YYYY');
to_char
-------------
22 Oct 2018
(1 row)
In your case query will be like this
SELECT
to_char(created, 'DD Mon YYYY') as day,
COUNT(*) as count
FROM users
WHERE created >= ${startDate} AND created <= ${endDate}
GROUP BY to_char(created, 'DD Mon YYYY')
Here you can find datetime format specification https://www.postgresql.org/docs/9.6/static/functions-formatting.html#FUNCTIONS-FORMATTING-DATETIME-TABLE