SQL grouping user count by Mondays - sql

Question

Given a Users table like so:
Users: id, created_at
How can I get the # of users created grouped by day? My goal is to see the number of users created this Monday versus previous Monday's.

Answer 1

If created_at is of type timestamp, the simplest and fastest way is a plain cast to date:
SELECT created_at::date AS day, count(*) AS ct
FROM users
GROUP BY 1;
Since I am assuming that id cannot be NULL, count(*) is a tiny bit shorter and faster than count(id), while doing the same.
If you just want to see days since "last Monday":
SELECT created_at::date, count(*) AS ct
FROM users
WHERE created_at >= (now()::date - (EXTRACT(ISODOW FROM now())::int + 6))
GROUP BY 1
ORDER BY 1;
This is carefully drafted to use a sargable condition, so it can use a simple index on created_at if present.
Consider the manual for EXTRACT.

Answer 2

SELECT COUNT(id) AS cnt, EXTRACT(DOW FROM created_at) AS dow
FROM Users
GROUP BY EXTRACT(DAY FROM created_at)

Answer 3

If you want to see the days, use to_char(<date>, 'Day').
So, one way to do what you want:
select date_trunc('day', created_at), count(*)
from users u
where to_char(created_at, 'Dy') = 'Mon'
group by date_trunc('day', created_at)
order by 1 desc;
Perhaps a more general way to look at it would be to summarize the results by day of the week, for this week and last week. Something like:
select to_char(created_at, 'Day'),
sum(case when created_at >= current_date - 6 then 1 else 0 end) as ThisWeek,
sum(case when trunc(created_at) between current_date - 13 and current_date - 7 then 1 else 0 end) as LastWeek
from users u
group by to_char(created_at, 'Day')

Answer 4

I am from a T-SQL background and I would do something like this
CREATE TABLE #users
(id int,
created_at datetime
)
INSERT INTO #users
(id, created_at)
VALUES
(
1, getdate()
)
INSERT INTO #users
(id, created_at)
VALUES
(
1, getdate()
)
INSERT INTO #users
(id, created_at)
VALUES
(
1, dateadd(DAY, 1,getdate())
)
SELECT id, created_at, count(id) FROM #users
GROUP BY id, created_at
DROP TABLE #users
You will get better results if you only group by day part and not the entire datetime value.
Coming to second part - only comparing for Mondays; you can use something like
select datename(dw,getdate())
the above will give you the name of the weekday which you can compare against a string literal 'Monday'.