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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