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!