( SELECT DISTINCT sender_user_id, receiver_user_id FROM user_message WHERE DATE( create_time ) = CURDATE() AND back_user_id IS NULL ) AS distinct_pairs
SELECT COUNT(*) AS count_distinct_pairs
FROM (SELECT DISTINCT sender_user_id, receiver_user_id
FROM user_message
WHERE DATE (create_time) = CURDATE() AND back_user_id IS NULL ) AS distinct_pairs