BigQuery: Join based on closest timestamp

问题

I am joining two tables based on closest timestamp in BigQuery and getting this error. Unsupported subquery with table in join predicate.

SELECT gs.user_session_id
      ,dtc._date
      ,dtc.hit_timestamp _timestamp
      ,dtc.user_id
FROM  ga.2_deduped_twice_click_data dtc LEFT JOIN ga.sessions gs ON dtc.user_id = gs.user_id 
and dtc.hit_timestamp = ( SELECT dtc2.hit_timestamp FROM ga.2_deduped_twice_click_data dtc2 order by ABS(TIMESTAMP_DIFF(dtc.hit_timestamp, gs._timestamp, MILLISECOND)) LIMIT 1 )

回答1:

You could try a mix of ARRAY_AGG and ORDER BY diff LIMIT 1:

WITH a AS (
  SELECT * FROM UNNEST(
    [STRUCT(TIMESTAMP('2018-01-02 20:01:00') AS time, 'monkey' AS animal)
     ,STRUCT('2018-03-04 10:10:10', 'lion')
     ,STRUCT('2018-07-04 10:10:10', 'donkey')
    ]) 
),
b AS  (
  SELECT * FROM UNNEST(
    [STRUCT(TIMESTAMP('2017-01-02 10:01:00') AS time, 'one' AS festival)
     ,STRUCT('2019-03-04 10:10:10', 'two')
     ,STRUCT('2018-07-04 10:10:10', 'three')
     ,STRUCT('2018-03-05 10:10:10', 'four')
    ]) 
)


SELECT b,
  (SELECT AS STRUCT * 
   FROM a 
   ORDER BY ABS(TIMESTAMP_DIFF(b.time, a.time, SECOND))
   LIMIT 1) closest
FROM b

来源：https://stackoverflow.com/questions/58170852/bigquery-join-based-on-closest-timestamp