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SELECT 
  fullVisitorId, 
  transactionId, 
  first_sourceMedium, 
  last_sourceMedium 
FROM ( 
  SELECT 
    fullVisitorId, 
    transactionId, 
    NTH_VALUE(trafficSource.source + " / " + trafficSource.medium, 1) 
      OVER ( 
        PARTITION BY fullVisitorId 
        ORDER BY visitStartTime 
        RANGE BETWEEN 30 * 24 * 60 * 60 PRECEDING AND CURRENT ROW -- Lookback Window: 30 days 
      ) as first_sourceMedium, 
    (trafficSource.source + " / " + trafficSource.medium) as last_sourceMedium, 
  FROM 
    ( 
    SELECT 
      fullVisitorId, 
      visitStartTime, 
      SPLIT(GROUP_CONCAT(hits.transaction.transactionId)) as transactionId, 
      trafficSource.source, 
      trafficSource.medium 
    FROM 
      TABLE_DATE_RANGE([75923065.ga_sessions_], TIMESTAMP('2014-05-29'), TIMESTAMP('2014-06-30')) -- Analysis Window 
    GROUP EACH BY fullVisitorId, visitStartTime, trafficSource.source, trafficSource.medium 
    ) 
) 
WHERE 
  NOT transactionId IS NULL 
GROUP BY 1,2,3,4