Question

I have a USERS table.

Each user has connections in a CONNECTIONS table.

Each connection has a datetime and some referenced properties like timezone, stored in a TZ reference table.

I’d like to select the userID, and the TimeZoneLabel for the first and the last connection. Even if a user has no connection (so NULL or anything else would be displayed)

Do something like :

Select USERS.id,
min(TZ.label),
max(TZ.label)

from USERS
join CONNECTION on USERS.id = CONNECTIONS.userid
join TZ on TZ.id = CONNECTIONS.tzid

group by USERS.id
order by max(CONNECTIONS.dateconn)

But I can’t achieve doing that. I’ve found articles on the net about that, but nothing works when I try. The example above does not work for the label, as there are no real min / max values but the one used on the first CONNECTION and the one used on the last one.

And I have many of these in my real request so I’d like to avoid too many sub-select.

Answer 1

Without the timezones:

SELECT 
       u.id             AS userId
     , MIN(c.dateconn)  AS firstConnectionDatetime
     , MAX(c.dateconn)  AS lastConnectionDateTime
FROM Users AS u
  LEFT JOIN Connection AS c
    ON u.id = c.userid    
GROUP BY u.id    
ORDER BY lastConnectionDateTime

With timezones (assuming Connection table has id as Primary Key):

SELECT 
       u.id             AS userId
     , ConMin.dateconn  AS firstConnectionDatetime
     , ConMax.dateconn  AS lastConnectionDateTime
     , TzMin.label      AS firstTimeZoneLabel
     , TzMax.label      AS lastTimeZoneLabel
FROM Users AS u
  LEFT JOIN Connection AS ConMax
    ON ConMax.id =
        ( SELECT c.id
          FROM Connection AS c
          WHERE u.id = c.userid 
          ORDER BY c.dateconn DESC
          LIMIT 1
        )
  LEFT JOIN TzMax
    ON TzMax.id = ConMax.tzid
  LEFT JOIN Connection AS ConMin
    ON ConMin.id =
        ( SELECT c.id
          FROM Connection AS c
          WHERE u.id = c.userid 
          ORDER BY c.dateconn ASC
          LIMIT 1
        )
  LEFT JOIN TzMin
    ON TzMin.id = ConMin.tzid

A compound (userid, dateconn, id) index on Connection table would help performance.