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 withnon-NULL values.  What I want is the most
recentrecord with non-NULL values*before* the record I'm trying to update.  In other words, if I have thefollowing
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 record1673.

> 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 programruns overnight.  I just can't afford to take
aprocedural approach and correctone record at a time ... there are 200,000 records and growing at a rate of8,000
recordsper 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 orcustom aggregates, something that would make
thiswhole process a lot easier.
 

Thanks for the help!

-Josh Berkus

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