获取上个月的Distinct用户总数，以SQL表示

SELECT COUNT(DISTINCT user_id) AS unique_users_last_month
FROM user_activity
WHERE activity_date >= DATE_TRUNC('month', CURRENT_DATE - INTERVAL '1 month')
  AND activity_date < DATE_TRUNC('month', CURRENT_DATE);

SELECT COUNT(DISTINCT user_id) AS unique_users_last_month
FROM user_activity
WHERE activity_date >= DATE_ADD(LAST_DAY(CURRENT_DATE - INTERVAL 1 MONTH) + INTERVAL 1 DAY, INTERVAL 0 MONTH)
  AND activity_date < LAST_DAY(CURRENT_DATE) + INTERVAL 1 DAY;

SELECT COUNT(DISTINCT user_id) AS unique_users_last_month
FROM user_activity
WHERE activity_date >= DATEADD(MONTH, DATEDIFF(MONTH, 0, DATEADD(MONTH, -1, GETDATE())), 0)
  AND activity_date < DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0);