SELECT user_id, MIN(activity_date) AS login_date
FROM Traffic
WHERE activity='login'
GROUP BY user_id;
2 再用90天筛选，并用你group by求数量
SELECT login_date, COUNT(*) AS user_count
FROM (
SELECT user_id, MIN(activity_date) AS login_date
FROM Traffic
WHERE activity='login'
GROUP BY user_id
) AS t
WHERE DATEDIFF("2019-06-30", login_date)<=90
GROUP BY login_date;