SQL Query to Join Two Tables Based Off Closest Timestamp

Question

I have two tables in SQL and I need to be able to do a join based off of the timestamp in table B that is earlier than or equal to the timestamp in table A.

So, here

Answer 1

If you can make changes to the table structures, I recommend changing the classification table to include an end date as well as a start date - it will be much easier to join to the table that way.

If not, I suggest the following:

SELECT case.id, case.resolution, case.timestamp, class.value
  FROM closed_cases AS case
  LEFT JOIN (select c.*, 
                    (select min(timestamp)
                     from classifications c1
                      where c1.timestamp > c.timestamp) timeend
             from classifications c) AS class 
  ON case.timestamp >= class.timestamp and 
     (case.timestamp < class.timeend or class.timeend IS NULL)
  WHERE case.timestamp BETWEEN $1 AND $2;

EDIT - with the end date on classification:

SELECT case.id, case.resolution, case.timestamp, class.value
  FROM closed_cases AS case
  LEFT JOIN classifications AS class 
  ON case.timestamp >= class.timestamp and case.timestamp < class.timeend
  WHERE case.timestamp BETWEEN $1 AND $2;