-- This SQL snippet attributes an interest to traffic source. Any interest 
-- event is attributed to previous traffic_source event. 

with 

-- Merge traffic_sources and interest events in one event pool. 
--  * pad with 'null' every field that is not common between these two events
--  * add a new field 'is_first_event'; set it to 1 for traffic_source events 
--  and to 0 for interest events
--  * add a new field 'type' with event type: interest or traffic_source.

merged as (
  select
    'traffic_source' as type,
    sent_at,
    user_id,
    name as traffic_source_name,
    campaign,
    null as interest_id,
    null as state,
    null as action,
    null as contract,
    null as source_app,
    1 as is_first_event
  from web.traffic_source

  union

  select
    'interest' as type,
    sent_at,
    user_id,
    null as traffic_source_name,
    null as campaign,
    interest_id,
    state,
    action,
    contract,
    source_app,
    0 as is_first_event
  from api.interest
  where 
    source_app in ('TabletApartmentList', 'ApartmentList', 'MobileApartmentList')
),

-- Add a new column with session_id per user. Partition table by user id,
-- and calculare running sum of is_first_event. This gives you a unique id. 

interest_with_session as (
  select 
    *, 
    sum(is_first_event) over 
      ( partition by user_id
        order by sent_at
        rows between unbounded preceding and current row ) as session_id
  from merged
),

-- Copy traffic_source campaign and name to all subsequent events. 
--   * Now partition by user_id, and session_id
--   * Use first_value() window function, to copy the value from first
-- event in partition to all other events. 

interest_with_attribution as (
  select 
    *,
    first_value(campaign) over 
      ( partition by user_id, session_id
        order by sent_at
        rows between unbounded preceding and current row) as interest_campaign,
    first_value(traffic_source_name) over 
      ( partition by user_id, session_id
        order by sent_at
        rows between unbounded preceding and current row) as interest_traffic_source
  from interest_with_session
)

-- Now filter out irrelevant columns, and filter keep only interest rows. 

select 
  sent_at,
  user_id,
  interest_id,
  state,
  action,
  contract,
  interest_traffic_source,
  interest_campaign
from interest_with_attribution
where type = 'interest'

;