T-SQL Query To Get Count Of Users Who Logged In Every Day In A Month

Question

I have an Azure SQL Database with a table Logins in this format:

UserId	LoginDate
ABC123	2020-07-10 19:41:51.1256874
ABC123	2020-07-11 15:52:43.5685147
XYZ789	2020-06-12 18:23:15.5524874

I need to write a query that will get the number of users that logged in every day in a month. I'm not really a SQL guy, so my first thought was to loop through the user ids and the dates in a date range and increment a counter, but I soon figured out that loops don't perform well in SQL. I have read some articles and posts about SQL being set-based, but I haven't figured out how to apply what I read to my problem. Any help would be appreciated!

Answer 1

You would convert the login date column to a date and aggregation:

select convert(date, logindate), count(distinct userid)
from t
group by convert(date, logindate);

Note that this uses count(distinct) . If a user logins in multiple times on the same day, the user only counts once.

You can add a where clause for a particular month. Something like this:

where logindate >= '2021-01-01' and logindate < '2021-02-01'

for January 2021.

EDIT:

If you want the number of users who logged in every day during a specific period, then:

select count(*)
from (select userid, convert(date, logindate) as the_date,
             count(*) over (partition by userid) as num_dates
      from t
      where logindate >= @start and
            logindate < dateadd(day, 1, @end)
      group by userid, convert(date, logindate)
     ) u
where num_dates = datediff(day, @end, @start) + 1;

Or:

select count(*)
from (select userid,
             count(distinct convert(date, logindate)) num_dates
      from t
      where logindate >= @start and
            logindate < dateadd(day, 1, @end)
      group by userid
     ) u
where num_dates = datediff(day, @end, @start) + 1;

Answer 2

You can group by user, filter the query for the month that you want and set the condition in the HAVING clause:

SElECT UserId 
FROM Logins
WHERE CONVERT(date, LoginDate) BETWEEN '2021-01-01' AND '2021-01-31' -- for Jan 2021
-- or with YEAR() and MONTH()
-- WHERE YEAR(LoginDate) = 2021 AND MONTH(LoginDate) = 1 
GROUP BY UserId
HAVING COUNT(DISTINCT CONVERT(date, LoginDate)) = DAY(EOMONTH(MIN(LoginDate)))

If you want only the number of users use COUNT() window function:

SElECT DISTINCT COUNT(*) OVER () counter 
FROM Logins
WHERE CONVERT(date, LoginDate) BETWEEN '2021-01-01' AND '2021-01-31' -- for Jan 2021
-- or with YEAR() and MONTH()
-- WHERE YEAR(LoginDate) = 2021 AND MONTH(LoginDate) = 1 
GROUP BY UserId
HAVING COUNT(DISTINCT CONVERT(date, LoginDate)) = DAY(EOMONTH(MIN(LoginDate)))