Oracle SQL - Select users between two date by month - sql

Question

I am learning SQL and I was wondering how to select active users by month, depending on their starting and ending date (both timestamp(6)). My table looks like this:
Cust_Num | Start_Date | End_Date
1 | 2018-01-01 | 2019-01-01
2 | 2018-01-01 | NULL
3 | 2019-01-01 | 2019-06-01
4 | 2017-01-01 | 2019-03-01
So, counting the active users by month, I should have an output like:
As of. | Count
2018-06-01 | 3
...
2019-02-01 | 3
2019-07-01 | 1
So far, I do a manual operation by entering each month:
Select
201906,
count(distinct a.cust_num)
From
active_users a
Where
to_date(‘20190630’,’yyyymmdd) between a.start_date and nvl (a.end_date, ‘31-dec-9999)
union all
Select
201905,
count(distinct a.cust_num)
From
active_users a
Where
to_date(‘20190531’,’yyyymmdd) between a.start_date and nvl (a.end_date, ‘31-dec-9999)
union all
...
Not very optimized and sustainable if I want to enter 10 years ao 120 months lol.
Any help is welcome. Thanks a lot!

Answer 1

This query shows the active-user-count effective as-of the end of the month.
How it works:
Convert each input row (with StartDate and EndDate value) into two rows that represent a point-in-time when the active-user-count incremented (on StartDate) and decremented (on EndDate). We need to convert NULL to a far-off date value because NULL values are sorted before instead of after non-NULL values:
This makes your data look like this:
OnThisDate Change
2018-01-01 1
2019-01-01 -1
2018-01-01 1
9999-12-31 -1
2019-01-01 1
2019-06-01 -1
2017-01-01 1
2019-03-01 -1
Then we simply SUM OVER the Change values (after sorting) to get the active-user-count as of that specific date:
So first, sort by OnThisDate:
OnThisDate Change
2017-01-01 1
2018-01-01 1
2018-01-01 1
2019-01-01 1
2019-01-01 -1
2019-03-01 -1
2019-06-01 -1
9999-12-31 -1
Then SUM OVER:
OnThisDate ActiveCount
2017-01-01 1
2018-01-01 2
2018-01-01 3
2019-01-01 4
2019-01-01 3
2019-03-01 2
2019-06-01 1
9999-12-31 0
Then we PARTITION (not group!) the rows by month and sort them by their date so we can identify the last ActiveCount row for that month (this actually happens in the WHERE of the outermost query, using ROW_NUMBER() and COUNT() for each month PARTITION):
OnThisDate ActiveCount IsLastInMonth
2017-01-01 1 1
2018-01-01 2 0
2018-01-01 3 1
2019-01-01 4 0
2019-01-01 3 1
2019-03-01 2 1
2019-06-01 1 1
9999-12-31 0 1
Then filter on that where IsLastInMonth = 1 (actually, where ROW_COUNT() = COUNT(*) inside each PARTITION) to give us the final output data:
At-end-of-month Active-count
2017-01 1
2018-01 3
2019-01 3
2019-03 2
2019-06 1
9999-12 0
This does result in "gaps" in the result-set because the At-end-of-month column only shows rows where the Active-count value actually changed rather than including all possible calendar months - but that's ideal (as far as I'm concerned) because it excludes redundant data. Filling in the gaps can be done inside your application code by simply repeating output rows for each additional month until it reaches the next At-end-of-month value.
Here's the query using T-SQL on SQL Server (I don't have access to Oracle right now). And here's the SQLFiddle I used to come to a solution: http://sqlfiddle.com/#!18/ad68b7/24
SELECT
OtdYear,
OtdMonth,
ActiveCount
FROM
(
-- This query adds columns to indicate which row is the last-row-in-month ( where RowInMonth == RowsInMonth )
SELECT
OnThisDate,
OtdYear,
OtdMonth,
ROW_NUMBER() OVER ( PARTITION BY OtdYear, OtdMonth ORDER BY OnThisDate ) AS RowInMonth,
COUNT(*) OVER ( PARTITION BY OtdYear, OtdMonth ) AS RowsInMonth,
ActiveCount
FROM
(
SELECT
OnThisDate,
YEAR( OnThisDate ) AS OtdYear,
MONTH( OnThisDate ) AS OtdMonth,
SUM( [Change] ) OVER ( ORDER BY OnThisDate ASC ) AS ActiveCount
FROM
(
SELECT
StartDate AS [OnThisDate],
1 AS [Change]
FROM
tbl
UNION ALL
SELECT
ISNULL( EndDate, DATEFROMPARTS( 9999, 12, 31 ) ) AS [OnThisDate],
-1 AS [Change]
FROM
tbl
) AS sq1
) AS sq2
) AS sq3
WHERE
RowInMonth = RowsInMonth
ORDER BY
OtdYear,
OtdMonth
This query can be flattened into fewer nested queries by using aggregate and window functions directly instead of using aliases (like OtdYear, ActiveCount, etc) but that would make the query much harder to understand.

Answer 2

I have created the query which will give the result of all the months starting from the minimum start date in the table till maximum end date.
You can change it using adding one condition in WHERE clause.
-- table creation
CREATE TABLE ACTIVE_USERS (CUST_NUM NUMBER, START_DATE DATE, END_DATE DATE)
-- data creation
INSERT INTO ACTIVE_USERS
SELECT * FROM
(
SELECT 1, DATE '2018-01-01', DATE '2019-01-01' FROM DUAL UNION ALL
SELECT 2, DATE '2018-01-01', NULL FROM DUAL UNION ALL
SELECT 3, DATE '2019-01-01', DATE '2019-06-01' FROM DUAL UNION ALL
SELECT 4, DATE '2017-01-01', DATE '2019-03-01' FROM DUAL
)
-- data in the actual table
SELECT * FROM ACTIVE_USERS ORDER BY CUST_NUM;
CUST_NUM START_DATE END_DATE
---------- ---------- ----------
1 2018-01-01 2019-01-01
2 2018-01-01
3 2019-01-01 2019-06-01
4 2017-01-01 2019-03-01
Query to fetch desired result
WITH CTE ( START_DATE, END_DATE ) AS
(
SELECT
ADD_MONTHS( START_DATE, LEVEL - 1 ),
ADD_MONTHS( START_DATE, LEVEL ) - 1
FROM
(
SELECT
MIN( START_DATE ) AS START_DATE,
MAX( END_DATE ) AS END_DATE
FROM
ACTIVE_USERS
)
CONNECT BY LEVEL <= CEIL( MONTHS_BETWEEN( END_DATE, START_DATE ) ) + 1
)
--
--
SELECT
C.START_DATE,
COUNT(1) AS CNT
FROM
CTE C
JOIN ACTIVE_USERS D ON
(
C.END_DATE BETWEEN
D.START_DATE
AND
CASE
WHEN D.END_DATE IS NOT NULL THEN D.END_DATE
ELSE C.END_DATE
END
)
GROUP BY
C.START_DATE
ORDER BY
C.START_DATE;
-- output --
START_DATE CNT
---------- ----------
2017-01-01 1
2017-02-01 1
2017-03-01 1
2017-04-01 1
2017-05-01 1
2017-06-01 1
2017-07-01 1
2017-08-01 1
2017-09-01 1
2017-10-01 1
2017-11-01 1
START_DATE CNT
---------- ----------
2017-12-01 1
2018-01-01 3
2018-02-01 3
2018-03-01 3
2018-04-01 3
2018-05-01 3
2018-06-01 3
2018-07-01 3
2018-08-01 3
2018-09-01 3
2018-10-01 3
START_DATE CNT
---------- ----------
2018-11-01 3
2018-12-01 3
2019-01-01 3
2019-02-01 3
2019-03-01 2
2019-04-01 2
2019-05-01 2
2019-06-01 1
30 rows selected.
Cheers!!