Question

I’ve got four tables: A, B, C, D. D has a FK to C, C has a FK to B, B has a FK to A. Each table has an ID column. In addition to ID and CID, table D has Value and Timestamp (an integer). Given a certain time, I need the row from D before the Timestamp but closest to it for each CID — Unless there are no rows before the Timestamp, in which case I need the first value after after the Timestamp. I’m using Sqlite.

Leaving out this last requirement, I attempted to get the values before the Timestamp with this query, but the ID returned is wrong:

SELECT  * 
FROM    D d 
INNER JOIN C c ON d.CID = c.ID 
INNER JOIN B b ON C.BID = b.ID
WHERE   b.AID = @AID AND d.Timestamp = 
        (
        SELECT  MAX(d2.Timestamp) 
        FROM    D d2 
        WHERE   d2.Timestamp < @StartTime 
                AND d2.CID = D.CID
        )

Answer 1

This query should get the last entry before @StartTime in the first part of the query (before the UNION ALL) then combine it with the first entry after @StartTime. All that remains is to pick the smallest entry for each CID.

SELECT d2.*
FROM (
  SELECT D.ID, MAX(D.Timestamp)
  FROM D d
    INNER JOIN C c ON c.ID = d.CID
      INNER JOIN B b on b.ID = c.BID
  WHERE b.AID = @AID AND d.Timestamp <= @StartTime 
  GROUP BY d.CID) AS results
INNER JOIN D d2 ON d2.ID = results.ID

UNION ALL 

SELECT d2.*
FROM (
  SELECT D.ID, MIN(D.Timestamp)
  FROM D d
    INNER JOIN C c ON c.ID = d.CID
      INNER JOIN B b on b.ID = c.BID
  WHERE b.AID = @AID AND d.Timestamp > @StartTime 
  GROUP BY d.CID) AS results
INNER JOIN D d2 ON d2.ID = results.ID