Question

I have a database in SQLITE (lets call it params) that stores a list of parameter changes and the times that they changed. ie:

key    paramid    time    val
---    -------    ----    ---
1      7          1.2     0
2      3          2.5     23
3      3          5.6     54
4      3          7.1     76
5      7          8.4     1
6      3          9.2     33
7      9          9.7     22
8      3          10.3    21

I would like to use a query to generate a table that includes every time one parameter changed along with the value of the other parameter when the first parameter changed and the time of the change. ie:

param3Val    latestParam7Val    time
---------    ---------------    ----
23           0                  2.5
54           0                  5.6
76           0                  7.1
33           1                  9.2
21           1                  10.3

Any ideas for doing this efficiently in SQL?

Thanks!

Answer 1

You could use:

SELECT p1.val
     , (SELECT p2.val
          FROM params p2
         WHERE p2.paramid = 7
           AND p2.time < p1.time
         ORDER BY p2.time DESC
         LIMIT 1
       ) latestParam7Val
     , p1.time
  FROM params p1

Alternative with joins:

SELECT p1.val
     , p2.val
     , p1.time
  FROM params p1
  LEFT JOIN params p2 ON p2.paramid = 7 AND p2.time < p1.time
  LEFT JOIN params p3 ON p3.paramid = 7 AND p3.time < p1.time
                                        AND p3.time > p2.time
 WHERE p3.time IS NULL