MySQL条件付き自己結合

ユーザーごとに最も早い「imp」が必要なようです。次に、その時点以降のトランザクションごとに集計できます。

SELECT dl.appid, SUM(dl.type = 'click') / SUM(dl.type = 'imp')
FROM dialoglog dl LEFT JOIN
     (SELECT dl2.userId, MIN(ds) as imp_ds
      FROM dialoglog dl2
      WHERE type = 'imp'
      GROUP BY dl2.userid
     ) dlimp
     ON dl.userid = dlimp.userid 
WHERE dl.ds >= dlimp.imp_ds
GROUP BY dl.appid;