Untitled

WITH
  aggregated_data AS (
  SELECT
    participant_id,
    session_id,
    DATETIME_TRUNC(time, MINUTE) AS interval_start,
    AVG(heart_rate) AS avg_heart_rate,
    AVG(rr_interval) AS avg_rri,
    AVG(temperature1) AS avg_skin_temp
  FROM (
    SELECT
      participant_id,
      session_id,
      time,
      heart_rate,
      NULL AS rr_interval,
      NULL AS temperature1
    FROM
      `clean_hop_heart_rate`
    UNION ALL
    SELECT
      participant_id,
      session_id,
      time,
      NULL,
      rr_interval,
      NULL
    FROM
      `clean_hop_rri`  
    UNION ALL
    SELECT
      participant_id,
      session_id,
      time,
      NULL,
      NULL,
      temperature1
    FROM
      `clean_hop_skin_temp` ) combined_data
  GROUP BY
    participant_id,
    session_id,
    interval_start )
SELECT
  participant_id,
  session_id,
  interval_start AS DateTime,
  avg_heart_rate,
  avg_rri,
  avg_skin_temp
FROM
  aggregated_data
ORDER BY
  participant_id,
  session_id,
  interval_start;