SQL window function with a where clause?

Question

I\'m trying to correlate two types of events for users. I want to see all event \"B\"s along with the most recent event \"A\" for that user prior to the \"A\" event. How would

Answer 1

Just tried Gordon's approach using PostgreSQL 9.5.4, and it complained that

FILTER is not implemented for non-aggregate window functions

which means using lag() with FILTER is not allowed. So I modified Gordon's query using max(), a different window frame, and CTE:

WITH subq AS (
  SELECT
    "user", event, time as event_b_time,
    max(time) FILTER (WHERE event = 'A') OVER (
      PARTITION BY "user"
      ORDER BY time
      ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
    ) AS last_event_a_time
  FROM events
  ORDER BY time
)
SELECT
  "user", event_b_time, last_event_a_time
FROM subq
WHERE event = 'B';

Verified that this works with PostgreSQL 9.5.4.

Thanks to Gordon for the FILTER trick!

Answer 2

There is not need for window functions here. Just find all B events, and for each one of them, find the most recent A of the same user via a subquery. Something like that should do it:

SELECT
    "user",
    time AS event_b_time,
    (SELECT time AS last_event_a_time
     FROM t t1
     WHERE "user"=t.user AND event='A' AND time<t.time
     ORDER BY time DESC LIMIT 1)
FROM t
WHERE event='B';

I assume that the table is called t (I used it twice).