Question 290676 in Mysql Question posted in
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MySQL Joining same table multiple times makes count go weird

Edited to clarify and simplify

I have the following query

select 
  Signups.user_id,
  Count(timeoutTable.user_id) as timeoutCount,
  Count(confirmedTable.user_id) as confirmedCount
from Signups
left join (
  select * from Confirmations where action = 'timeout'
) as timeoutTable on signups.user_id = timeoutTable.user_id
left join (
  select * from Confirmations where action = 'confirmed'
) as confirmedTable on signups.user_id = confirmedTable.user_id
group by 
  Signups.user_id,
  timeoutTable.user_id,
  confirmedTable.user_id

When run with the following input

Signups Table

| user_id | time_stamp          |
| ------- | ------------------- |
| 15      | 2020-07-31 18:26:35 |
| 16      | 2021-05-20 01:38:09 |
| 7       | 2020-08-02 08:45:14 |
| 10      | 2020-06-24 17:13:14 |
| 5       | 2020-06-27 17:59:29 |
| 9       | 2021-11-08 03:05:14 |
| 8       | 2021-12-13 03:38:58 |
| 12      | 2020-09-16 11:17:39 |


Confirmations Table

| user_id | time_stamp          | action    |
| ------- | ------------------- | --------- |
| 7       | 2020-03-31 13:11:43 | timeout   |
| 7       | 2021-03-25 07:40:25 | timeout   |
| 8       | 2020-07-27 19:43:25 | confirmed |
| 8       | 2021-03-07 19:48:06 | timeout   |
| 7       | 2020-01-24 15:43:47 | confirmed |

It outputs:

| user_id | timeoutCount | confirmedCount |
| ------- | ------------ | -------------- |
| 15      | 0            | 0              |
| 16      | 0            | 0              |
| 7       | 2            | 2              |
| 10      | 0            | 0              |
| 5       | 0            | 0              |
| 9       | 0            | 0              |
| 8       | 1            | 1              |
| 12      | 0            | 0              |

I am trying to make it so that user_id 7 has a confirmed of 1 and a timeout of 2 but it sets both values to 2 for some reason. Any help would be greatly appreciated.

Tags:

2

Answers


  1. 0

    Your problem is that you are joining all timeouts to all confirmations. So, for a signup with 2 timeouts and 3 confirmations, you’ll get 2 x 3 = 6 rows that you then aggregate.

    When aggregating more than one table, aggregate before joining:

    select 
  user_id, 
  round(case when total_messages = 0 then 0 else total_confirmed / total_messages end, 2) as confirmation_rate
from
(
  select 
    s.user_id, 
    t.total_timeouts,
    c.total_confirmed,
    coalesce(t.timeoutCount,0) + coalesce(c.total_confirmed, 0) as total_messages
  from signups s
  left join 
  (
    select user_id, count(*) as total_timeouts
    from confirmations
    where action = 'timeout'
    group by user_id
  ) as t on t.user_id = s.user_id
  left join 
  (
    select user_id, count(*) as total_confirmed
    from confirmations
    where action = 'confirmed'
    group by user_id
  ) as c on c.user_id = s.user_id
)
order by user_id;

    An alternative to this is conditional aggregation here:

    select 
  user_id, 
  round(case when total_messages = 0 then 0 else total_confirmed / total_messages end, 2) as confirmation_rate
from
(
  select 
    s.user_id, 
    sum(c.action = 'timeout') as total_timeouts,
    sum(c.action = 'confirmed') as total_confirmed,
    sum(c.action = 'timeout') - sum(c.action = 'confirmed') as total_messages
  from signups s
  left join confirmations c on c.user_id = s.user_id
  group by s.user_id
)
order by user_id;
    Login or Signup to reply.
  2. 0

    To see what’s happening, look at the subquery without the group by.

    select 
  Signups.user_id,
  timeoutTable.*,
  confirmedTable.*
from Signups
left join (
  select * from Confirmations where action = 'timeout'
) as timeoutTable on Signups.user_id = timeoutTable.user_id
left join (
  select * from Confirmations where action = 'confirmed'
) as confirmedTable on Signups.user_id = confirmedTable.user_id

                timeoutTable                                    confirmedTable
user_id     user_id     time_stamp              action      user_id     time_stamp              action
15          null        null                    null        null        null                    null
16          null        null                    null        null        null                    null
7           7           2021-03-25 07:40:25     timeout     7           2020-01-24 15:43:47     confirmed
7           7           2020-03-31 13:11:43     timeout     7           2020-01-24 15:43:47     confirmed
10          null        null                    null        null        null                    null
5           null        null                    null        null        null                    null
9           null        null                    null        null        null                    null
8           8           2021-03-07 19:48:06     timeout     8           2020-07-27 19:43:25     confirmed
12          null        null                    null        null        null                    null

    Note how user_id 7 has 2 rows and each have a timeout and a confirmation. These rows are counted as both a timeout and a confirmation. This is because you’re joining with two tables at the same time.

    If we instead do the aggregations first and then join, the counts work out.

    select 
  Signups.user_id,
  timeoutTable.*,
  confirmedTable.*
from Signups
left join (
  select user_id, count(user_id) as timeouts
  from Confirmations
  where action = 'timeout'
  group by user_id
) as timeoutTable on Signups.user_id = timeoutTable.user_id
left join (
  select user_id, count(user_id) as confirmations
  from Confirmations
  where action = 'confirmed'
  group by user_id
) as confirmedTable on Signups.user_id = confirmedTable.user_id

                timeoutTable            confirmedTable
user_id     user_id     timeouts    user_id     confirmations
15          null        null        null        null
16          null        null        null        null
7           7           2           7           1
10          null        null        null        null
5           null        null        null        null
9           null        null        null        null
8           8           1           8           1
12          null        null        null        null

    Demonstration

    Note:

    left join (
  select * from Confirmations where action = 'timeout'
) as timeoutTable on Signups.user_id = timeoutTable.user_id

    is better written as without the subquery.

    left join Confirmations as timeoutTable
  on Signups.user_id = timeoutTable.user_id
 and where action = 'timeout'
    Login or Signup to reply.
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