Query to get user count for every month and previous month - sql

Question

We have the following activity table and would like to query it to get the number of unique users for each month and the previous month. The date field (createdat) is a timestamp. The query needs to work in PostgreSQL.
Activity table:
| id | userid | createdat | username |
|--------|--------|-------------------------|----------------|
| 1d658a | 4957f3 | 2016-12-06 21:16:35:942 | Tom Jones |
| 3a86e3 | 684edf | 2016-12-03 21:16:35:943 | Harry Smith |
| 595756 | 582107 | 2016-12-26 21:16:35:944 | William Hanson |
| 2c87fe | 784723 | 2016-12-07 21:16:35:945 | April Cordon |
| 32509a | 4957f3 | 2016-12-20 21:16:35:946 | Tom Jones |
| 72e703 | 582107 | 2017-01-01 21:16:35:947 | William Hanson |
| 6d658a | 582107 | 2016-12-06 21:16:35:948 | William Hanson |
| 5c077c | 5934c4 | 2016-12-06 21:16:35:949 | Sandra Holmes |
| 92142b | 57ea5c | 2016-12-15 21:16:35:950 | Lucy Lawless |
| 3dd0a6 | 5934c4 | 2016-12-04 21:16:35:951 | Sandra Holmes |
| 43509a | 4957f3 | 2016-11-20 21:16:35:946 | Tom Jones |
| 85142b | 57ea5c | 2016-11-15 21:16:35:950 | Lucy Lawless |
| 7c87fe | 784723 | 2017-1-07 21:16:35:945 | April Cordon |
| 9c87fe | 784723 | 2017-2-07 21:16:35:946 | April Cordon |
Results:
| Month | UserThis Month | UserPreviousMonth |
|----------|----------------|-------------------|
| Dec 2016 | 6 | 2 |
| Jan 2017 | 2 | 6 |
| Feb 2017 | 1 | 2 |

Answer 1

You can try this query. to_char to get MON YYYY, You can try to write a subquery with lag windows function to get UserPreviousMonth count.
SELECT *
FROM (SELECT To_char(createdat, 'MON YYYY') Months,
Count(DISTINCT username) UserThisMonth,
Lag(Count(DISTINCT username)) OVER (
ORDER BY Date_part('year', createdat),
Date_part('month',createdat)
) UserPreviousMonth
FROM t
GROUP BY Date_part('year', createdat),
To_char(createdat, 'MON YYYY'),
Date_part('month', createdat)) t
WHERE userpreviousmonth IS NOT NULL
sqlfiddle:http://sqlfiddle.com/#!15/45e52/2
| months | userthismonth | userpreviousmonth |
|----------|---------------|-------------------|
| DEC 2016 | 6 | 2 |
| JAN 2017 | 2 | 6 |
| FEB 2017 | 1 | 2 |
EDIT
Types of Dec 2016 and Jan 2017 ... must string, because DateTime need a full date like 2017-01-01. If you need to be sorted and used on the graph I will suggest you sort on this query years and months columns, then make date string on front-end.
SELECT *
FROM (SELECT Date_part('year', createdat) years,
Date_part('month', createdat) months,
Count(DISTINCT username) UserThisMonth,
Lag(Count(DISTINCT username)) OVER (
ORDER BY Date_part('year', createdat),
Date_part('month',createdat)
) UserPreviousMonth
FROM user_activity
GROUP BY Date_part('year', createdat),
Date_part('month', createdat)) t
WHERE userpreviousmonth IS NOT NULL
sqlfiddle:http://sqlfiddle.com/#!15/2da2b/4
| years | months | userthismonth | userpreviousmonth |
|-------|--------|---------------|-------------------|
| 2016 | 12 | 6 | 2 |
| 2017 | 1 | 2 | 6 |
| 2017 | 2 | 1 | 2 |

Answer 2

Fastest and simplest with date_trunc(). Use to_char() once to display the month in preferred format:
WITH cte AS (
SELECT date_trunc('month', createdat) AS mon
, count(DISTINCT username) AS ct
FROM activity
GROUP BY 1
)
SELECT to_char(t1.mon, 'MON YYYY') AS month
, t1.ct AS users_this_month
, t2.ct AS users_previous_month
FROM cte t1
LEFT JOIN cte t2 ON t2.mon = t1.mon - interval '1 mon'
ORDER BY t1.mon;
db<>fiddle here
You commented:
the "Month" field in the results table needs to be a "date" data type so it can be sorted and used on the graph.
For this, simply cast in the final SELECT:
SELECT t1.mon::date AS month ...
Grouping and ordering by a (truncated) timestamp value is more efficient (and reliable) than by multiple values or a text representation.
The result includes the first month ('NOV 2016' in your demo), showing NULL for users_previous_month - like for any previous month without entries. You might want to display 0 instead or drop the row ...
Related:
How to get the date and time from timestamp in PostgreSQL select query?
PostgreSQL: running count of rows for a query 'by minute'
Aside: usernames in the form of "Tom Jones" are typically not unique. You'll want to operate with a unique ID instead.

Answer 3

Edit:
Shamelessly using #D-Shih's superior method of generating year/month combinations.
A couple of solutions:
WITH ua AS (
SELECT
TO_CHAR(createdate, 'YYYYMM') AS year_month,
COUNT(DISTINCT userid) distinct_users
FROM user_activity
GROUP BY
TO_CHAR(createdate, 'YYYYMM')
)
SELECT * FROM (
SELECT
TO_DATE(ua.year_month || '01', 'YYYYMMDD')
+ INTERVAL '1 month'
- INTERVAL '1 day'
AS month_end,
ua.distinct_users,
LAG(ua.distinct_users) OVER (ORDER BY ua.year_month) distinct_users_last_month
FROM ua
) uas WHERE uas.distinct_users_last_month IS NOT NULL
ORDER BY month_end DESC;
No windowing required:
WITH ua AS (
SELECT
TO_CHAR(createdate, 'YYYYMM') AS year_month,
TO_CHAR(createdate - INTERVAL '1 MONTH', 'YYYYMM') AS last_month,
COUNT(DISTINCT userid) AS distinct_users
FROM user_activity
GROUP BY
TO_CHAR(createdate, 'YYYYMM'),
TO_CHAR(createdate - INTERVAL '1 MONTH', 'YYYYMM')
)
SELECT
TO_DATE(ua1.year_month || '01', 'YYYYMMDD')
+ INTERVAL '1 month'
- INTERVAL '1 day'
AS month_end,
ua1.distinct_users,
ua2.distinct_users AS last_distinct_users
FROM
ua ua1 LEFT OUTER JOIN ua ua2
ON ua1.year_month = ua2.last_month
WHERE ua2.distinct_users IS NOT NULL
ORDER BY ua1.year_month DESC;
DDL:
CREATE TABLE user_activity (
id varchar(50),
userid varchar(50),
createdate timestamp,
username varchar(50)
);
COMMIT;
Data:
INSERT INTO user_activity VALUES ('1d658a','4957f3','20161206 21:16:35'::timestamp,'Tom Jones');
INSERT INTO user_activity VALUES ('3a86e3','684edf','20161203 21:16:35'::timestamp,'Harry Smith');
INSERT INTO user_activity VALUES ('595756','582107','20161226 21:16:35'::timestamp,'William Hanson');
INSERT INTO user_activity VALUES ('2c87fe','784723','20161207 21:16:35'::timestamp,'April Cordon');
INSERT INTO user_activity VALUES ('32509a','4957f3','20161220 21:16:35'::timestamp,'Tom Jones');
INSERT INTO user_activity VALUES ('72e703','582107','20170101 21:16:35'::timestamp,'William Hanson');
INSERT INTO user_activity VALUES ('6d658a','582107','20161206 21:16:35'::timestamp,'William Hanson');
INSERT INTO user_activity VALUES ('5c077c','5934c4','20161206 21:16:35'::timestamp,'Sandra Holmes');
INSERT INTO user_activity VALUES ('92142b','57ea5c','20161215 21:16:35'::timestamp,'Lucy Lawless');
INSERT INTO user_activity VALUES ('3dd0a6','5934c4','20161204 21:16:35'::timestamp,'Sandra Holmes');
INSERT INTO user_activity VALUES ('43509a','4957f3','20161120 21:16:35'::timestamp,'Tom Jones');
INSERT INTO user_activity VALUES ('85142b','57ea5c','20161115 21:16:35'::timestamp,'Lucy Lawless');
INSERT INTO user_activity VALUES ('7c87fe','784723','20170107 21:16:35'::timestamp,'April Cordon');
INSERT INTO user_activity VALUES ('9c87fe','784723','20170207 21:16:35'::timestamp,'April Cordo');
COMMIT;