count(*) records grouped by date across two tables

Question

I've got two tables:

emailLog table:

email           sendTime                   sourceTag
-------------------------------------------------------
yadda@aol.com   2016-11-17 09:14:37.213    WelcomeEmail
badda@aol.com   2016-11-16 09:14:37.213    WelcomeEmail
test@aol.com    2016-11-15 09:12:33.213    WelcomeEmail

registrations table:

email           dateRegistered             regSource
-------------------------------------------------------
yadda@aol.com   2016-11-17 09:14:37.213    WelcomeEmail
badda@aol.com   2016-11-16 09:14:37.213    WelcomeEmail
test@aol.com    2016-11-15 09:12:33.213    WelcomeEmail

I'm trying to do a combined query to show the COUNT() of people who received an email on a given date compared with the COUNT() of people who have registered on a given date

I have gotten as far as this:

SELECT
    CONVERT(varchar(10), sendTime, 120) as date,
    COUNT(*) as numberSent 
    FROM emailLog
WHERE sourceTag = 'WelcomeEmail'
AND
sendTime BETWEEN '20161110' and '20161120'
GROUP BY
    CONVERT(varchar(10), sendTime, 120)
ORDER BY DATE ASC;

Which gives me a list of the emails sent with a specific sourceTag, grouped by the date:

DATE                NUMBERSENT
2016-11-17          256
2016-11-18          136
2016-11-19          40
2016-11-20          118
2016-11-21          186

But I can't figure out how to join a sum of the registrations for that date + with that source, like:

DATE                NUMBERSENT    MEMBERSREGISTERED
2016-11-17          256           12
2016-11-18          136           24
2016-11-19          40            13
2016-11-20          118           2
2016-11-21          186           11

I have tried to do something like...

SELECT 
 (SELECT count(*) from emailLog) 
    as emailLogResults, 
 (select count(*) from registrations) 
    as registrationResults
    ...

but I am stuck after that. Any help very much appreciated

Answer 1

To keep things simple and clear, i think, you can create 2 temporary tables. The first one will contain the list of the emails sent with a specific sourceTag, grouped by the date, as you already did. And the second table will do the same thing for the registrations. You can than inner join the 2 temporary tables.

Answer 2

You can use a full join (to get rows from the other table when are no rows for a specific date in either of the tables) with the two tables and then use conditional aggregation.

SELECT
    COALESCE(CONVERT(varchar(10), e.sendTime, 120),CONVERT(varchar(10), r.dateRegistered, 120)) as date,
    COUNT(e.email) as numberSent,
    COUNT(r.email) as membersRegistered
FROM emailLog e
FULL JOIN registrations r ON e.sendTime = r.dateRegistered
AND e.sourceTag = 'WelcomeEmail'
AND r.regSource = 'WelcomeEmail'
AND e.sendTime BETWEEN '20161110' and '20161120' 
AND r.dateRegistered BETWEEN '20161110' and '20161120'
GROUP BY COALESCE(CONVERT(varchar(10), e.sendTime, 120),CONVERT(varchar(10), r.dateRegistered, 120))
ORDER BY DATE ASC