Question

I’ve been racking my brain about how to do this in one query without PHP code.

In a nutshell, I have a table that records email activity. For the sake of this example, here is the data:

recipient_id     activity     date
1                delivered    2011-08-30
1                open         2011-08-31
2                delivered    2011-08-30
3                delivered    2011-08-24
3                open         2011-08-30
3                open         2011-08-31

The goal: I want to display to users a single number that tells how many recipients open their email within 24 hours.

E.G. “Users that open their email within 24 hours: 13 Readers”

In the case of the sample data, above, the value would be “1”. (Recipient one was delivered an email and opened it the next day. Recipient 2 never opened it and recipient 3 waited 5 days.)

Can anyone think of a way to express the goal in a single query?

Reminder: In order to count, the person must have a ‘delivered’ tag and at least one ‘open’ tag. Each ‘open’ tag only counts once per recipient.

** EDIT ** Sorry, I’m using MySQL

Answer 1

Here is a version in mysql.

select count(distinct recipient_id)
from email e1
where e1.activity = 'delivered'
  and exists
        (select * from email e2
         where e1.recipient_id = e2.recipient_id
           and e2.activity = 'open'
           and datediff(e2.action_date,e1.action_date) <= 1)

The basic principle is that you want to find a delivered row for a recipient that also has an open within 24 hours.

The datediff() is a good way to do the date arithmetic in mysql — other dbs will vary on exact methods for this step. The rest of the sql will work anywhere.

SQLFiddle here: http://sqlfiddle.com/#!2/c9116/4