At 10:55 AM 6/21/06, Worky Workerson wrote:
I'm having a bit of a brain freeze and can't seem to come up with
decent SQL for the following problem:
I have a table "t" of the form "time_occurred TIMESTAMP, prog_data
VARCHAR" and would like to create a query that outputs something of
the form "first_seen, last_seen, count, prog_data".
I have the current query which gets the first_seen and last_seen via
subqueries, ala
SELECT t1.time_occurred AS first_seen, t2.time_occurred AS last_seen,
t3.count, t1.prog_data
FROM t AS t1, t AS t2
WHERE t1.prog_data = t2.prog_data
AND t1.time_occurred IN (SELECT min(time_occurred) FROM t WHERE
prog_data = t1.prog_data)
AND t2.time_occurred IN (SELECT max(time_occurred) FROM t WHERE
prog_data = t1.prog_data)
but I can't seem to work out how to get the count of all the records
that have. I figure that this is probably a relatively common idiom
... can anyone suggest ways to go about doing this. Also, the
performance of this is pretty horrible, but I figure that creating a
column on t.prog_data should speed things up noticably, right?
Is this what you're looking for?
SELECT min(time_occurred) AS first_seen, max(time_occurred) AS last_seen,
count(*), prog_data from t group by prog_data;
Since this query has no WHERE or HAVING clause, this query will read the
entire table. There is nothing you can do to speed it up. If you have
enough RAM to hold the entire table (and appropriate setting to utilize
it), then a second (and subsequent) run of the query will be faster than
the first, but that's as good as it gets.
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