我有两张table：活动和会议。
活动：

+-----------+---------------------+------+------------+
| event_id  |      timestamp      | flag | session_id |
+-----------+---------------------+------+------------+
| kj123123j | 2020-01-01 22:51:11 |    0 |          1 |
| j24hjk234 | 2020-01-01 21:11:00 |    0 |          1 |
| kjh234khj | 2020-01-01 21:44:17 |    1 |          1 |
| 342hj24j3 | 2020-01-01 08:11:00 |    0 |          2 |
| kk1k12323 | 2020-01-01 13:55:12 |    1 |          2 |
| 890fd8sdf | 2020-01-01 20:55:14 |    0 |          2 |
+-----------+---------------------+------+------------+

会议：

+------------+---------+
| session_id | user_id |
+------------+---------+
|          1 | 12kk    |
|          2 | 44qj    |
+------------+---------+

我想要得到的是一个表，它统计每个用户在标志发生之前的事件。

+---------+-------+
| user_id | count |
+---------+-------+
| 12kk    |     1 |
| 44qj    |     1 |
+---------+-------+

我尝试了两种方法：
self-join表，我无法测试它，因为它非常慢（events表非常大）
2.

WITH
  events AS (
  SELECT
    events.event_id,
    events.timestamp,
    events.user_id
  FROM
    db.events events
  LEFT JOIN
    db.users users
  ON
    events.session_id = users.session_id),
  flags AS (
  SELECT
    events.event_id,
    events.timestamp
  FROM
    db.events events
  WHERE
   events.flag is TRUE )
SELECT
  events.user_id,
  SUM(CASE
      WHEN events.timestamp < flags.timestamp THEN 1
    ELSE
    0
  END
    )
FROM
  flags
JOIN
  events
ON
  events.event_id = flags.event_id
GROUP BY
  events.user_id

第二种方法的问题是count列只有0s，这是绝对不可能的。
能帮我解决这个问题吗？