Database: Select last non-null entries

Question

Here\'s a question I\'ve been racking my brain over. Let\'s say I have a table that has a series of timestamps and a part number as the primary key. The table stores incremental

Answer 1

Rather than using a UNION, it sounds like you really want subqueries in the field list. That is, instead of (SELECT ...) UNION (SELECT ...) UNION (SELECT ...), you want SELECT (SELECT ...), (SELECT ...), (SELECT ...).

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For example:

SELECT part,
       ( SELECT x_pos
           FROM part_changes
          WHERE part = pc.part
            AND x_pos IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS x_pos,
       ( SELECT y_pos
           FROM part_changes
          WHERE part = pc.part
            AND y_pos IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS y_pos,
       ( SELECT status
           FROM part_changes
          WHERE part = pc.part
            AND status IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS status
  FROM ( SELECT DISTINCT
                part
           FROM part_changes
       ) AS pc
;

But at this point I would really consider writing a stored procedure.

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Alternatively:

SELECT DISTINCT
       part,
       FIRST_VALUE(x_pos) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN x_pos IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS x_pos,
       FIRST_VALUE(y_pos) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN y_pos IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS y_pos,
       FIRST_VALUE(status) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN status IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS status
  FROM part_changes
;

Answer 2

ruakh is right. Alternative: Write a user-defined aggregate using SQL-CLR. This aggregate can run bottom-top-top over your rows and remember the first non-null value of each column.

You can also do this in the application. Make your program request rows in sorted order in batches of, say, 10 rows. Aggregate these batches like described above. If after the current batch is done a null-column is there get the next batch.

Answer 3

For only one part this should give you an answer .. thanks to ruakh

But I dont like this version ..

SELECT 
    (SELECT timestamp  FROM part_changes WHERE part = $part 
    ORDER BY timestamp DESC
    LIMIT 1) as timestamp,

    (SELECT x-pos FROM part_changes WHERE part = $part and x-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1) as xpos,

    (SELECT y-pos FROM part_changes WHERE part = $part and  y-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1) as ypos,

    (SELECT status FROM part_changes WHERE part = $part and status IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1)) as status

Answer 4

list of relevant timestamps:

select max timestamp from part_changes where x_POS is not null group by part

You can make this a view: lets call this view1

SELECT part_changes.part, part_changes.x-pos 
FROM part_changes left join view1 on part_changes.part = view1.part
WHERE x-pos IS NOT NULL 
 AND part_changes.timestamp = view1.timestamp 
GROUP BY part_changes.part

See where I am going? That should give you the full list for x-pos.