Question

I need to make a graph from a log. The log entries are not in regular intervals.

I would like to select rows between dates along with what the values were immediately before the start date (that is, from whenever the immediatly preceeding log was entered).

So, let’s say:

table Foo has id and value columns,

table Bar has id, foo_id, and value columns, and

table BarLog has id, foo_id, bar_id, bar_value and timestamp.

So there can be many Bars for one Foo.

I need all rows from BarLog for all Bars given some foo_id between, say, 07/01/2012 and 07/31/2012 and the value (row) for each Bar as it was on 07/01/2012.

Hope that made sense, if not, I’ll try to clarify.

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EDIT (above left for context):

Let’s simplify this down another step. If I have a table with two foreign keys, fk_a and fk_b, and a timestamp, how can I get the most recent rows with a given fk_a and a distict fk_b.

As suggested, here’s an example.

+----+------+------+-------------+
| id | fk_a | fk_b |  timestamp  |
+----+------+------+-------------+
| 1  | 1    | 1    | 01-JUL-2012 |
| 2  | 2    | 2    | 02-JUL-2012 |
| 3  | 1    | 1    | 04-JUL-2012 |
| 4  | 2    | 2    | 05-JUL-2012 |
| 5  | 1    | 3    | 07-JUL-2012 |
+----+------+------+-------------+

Given a fk_a of 1, I would want rows 3 and 5. So looking only at rows 1, 3, and 5 (those with fk_a of 1), get the most recent of each fk_b (where row 3 is more recent than row 1 for fk_b=1).

Thanks again.

Answer 1

Finally found this question which had what I was looking for. Basically just joining with a grouped select. So the answer for my edit would be something like

SELECT * FROM SomeTable a
JOIN (
    SELECT fk_b, MAX(timestamp) as latest
    FROM SomeTable
    GROUP BY fk_b
) b
ON a.id = b.id
WHERE a.fk_a = @someIdA

Which would return the latest of each distinct fk_b with a specified fk_a

The original question would just be a union of this with a simple get between dates