Referencing a column in a sub-query's where clause

Question

I have a table that has records for multiple session events. Each row is an event in a session, and a session can have multiples of the same event. Those are basically game sessions and each event is round start or round end. My data looks something like

Session_id | Event_type | Event_Time
1 | round_start | 12:01:00
1 | round_end | 12:02:00
1 | round_start| 12:05:00
1 | round_end | 12:7:00
2 | round_start | 14:11:00
2 | round_end | 14:12:00
3 | round_start| 15:09:00
3 | round_end | 15:13:00

I am trying to find the average round duration. I tried the following SQL

select
RS.session_id,
RS.Event_Time as StartTime,
RE.EndTime,
TIMESTAMPDIFF(MINUTE,RE.EndTime,RS.Event_Time) as duration
from amp_event_mamlaka as RS
left join 
(
    select session_id, min(event_time) as EndTimd from amp_event_mamlaka 
    where Event_Type = "Round End" and session_id = RS.session_id and  event_time>RS.Event_Time
) RE
on RE.session_id = RS.session_id

The issue is that I can't reference RS.session_id and RS.event_time in the joined table.

I am using MySQL. Any suggestions on how to accomplish this?

Thanks

Answer 1

I would suggest that you approach this with a correlated subquery:

select RS.session_id, RS.Event_Time as StartTime,
       (select smin(event_time) 
        from amp_event_mamlaka em
        where em.session_id = RS.session_id and
              em.Event_Type = 'Round End' and 
              em.event_time > RS.Event_Time
       ) as EndTime,
from amp_event_mamlaka RS;

You can do the timestamp difference using a subquery:

select RS.*, TIMESTAMPDIFF(MINUTE, EndTime, Event_Time) as duration
from (select RS.session_id, RS.Event_Time as StartTime,
             (select min(event_time) 
              from amp_event_mamlaka em
              where em.session_id = RS.session_id and
                    em.Event_Type = 'Round End' and 
                    em.event_time > RS.Event_Time
             ) as EndTime
      from amp_event_mamlaka RS
     ) RS

Answer 2

Instead of keeping it in where clause of subquery you can keep the condition in Join On clause. Try this.

SELECT RS.session_id,
       RS.Event_Time                                    AS StartTime,
       RE.EndTime,
       Timestampdiff(MINUTE, RE.EndTime, RS.Event_Time) AS duration
FROM   amp_event_mamlaka AS RS
       LEFT JOIN (SELECT session_id,
                         Min(event_time) AS EndTimd
                  FROM   amp_event_mamlaka
                  WHERE  Event_Type = "Round End") RE
              ON RE.session_id = RS.session_id
                 AND RE.event_time > RS.Event_Time 

Answer 3

A subquery, as opposed to a nested query, should only return one value. Your requirement is an example where you want data from pairs of rows. The subquery is only used to connect the pair, not supply data. Fiddle

select  e1.SessionID, e1.EventType, e1.EventTime, e2.EventType, e2.EventTime, TimeStampDiff( minute, e1.EventTime, e2.EventTime ) Duration
from    Events  e1
join    Events e2
    on  e2.SessionID    = e1.SessionID
    and e2.EventType    = 'end'
    and e2.EventTime    =(
        select  Min( EventTime )
        from    Events
        where   SessionID   = e1.SessionID
            and EventType   = 'end'
            and EventTime   > e1.EventTime )
where   e1.EventType = 'start';