Re: UPDATE Query problem

Tom,

> Sure you can.  You can't alias history in the UPDATE, but you can alias
> it in the subselect, so:
> 
> UPDATE history SET fieldA =
>   (SELECT fieldA FROM history older
>    WHERE older.key = history.key AND
>    older.fieldA IS NOT NULL AND older.fieldB IS NOT NULL AND
>    older.timestamp =
>      (SELECT max(timestamp) FROM history oldest
>       WHERE oldest.key = history.key AND
>       oldest.fieldA IS NOT NULL AND oldest.fieldB IS NOT NULL)),
> fieldB = (SELECT fieldB FROM ... repeat entire subselect above ...)
> WHERE (history.fieldA IS NULL or 
>        history.fieldB IS NULL);

Interesting.  however, it appears to give me the most recent record with
 non-NULL values.  What I want is the most recent record with non-NULL values
 *before* the record I'm trying to update.  In other words, if I have the
 following data:

history
id	timestamp	fieldA	fieldB
1341	6/30/00		KCKG	1
1345	7/31/00		KC	1
1402	8/31/00		NULL	NULL
2799	9/30/00		NULL	NULL
1581	10/31/00	KC	2
1673	11/30/00	KC	2

I want records 1402 and 2799 to be updated from record 1345, not from record
 1673.

> This will work and (AFAIK) is fully SQL-compliant, but it will be
> slower than the dickens because of all those subselects :-(.  Might
> be tolerable if the key field is near-unique and is indexed, but
> heaven help you if not.

The key field is unique.  And slow is OK ... the history-correction program
 runs overnight.  I just can't afford to take a procedural approach and correct
 one record at a time ... there are 200,000 records and growing at a rate of
 8,000 records per month.

> Also, just because PG can handle the above doesn't mean every RDBMS does
> (do I need to name names?).  What products do you really need it to
> be portable to?

Yeah, you guessed it ... MS SQL Server 7.  Which kills custom functions or
 custom aggregates, something that would make this whole process a lot easier.

Thanks for the help!

-Josh Berkus

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