Re: Can someone help me with multiple "Left Outer Joins"?

Steve (budgethelp@xxxxxxxxx) writes:
> I have a SQL query I'm invoking via VB6 & ADO 2.8, that requires three
> "Left Outer Joins" in order to return every transaction for a specific
> set of criteria.
>
> Using three "Left Outer Joins" slows the system down considerably.
>
> I've tried creating a temp db, but I can't figure out how to execute
> two select commands. (It throws the exception "The column prefix
> 'tempdb' does not match with a table name or alias name used in the
> query.")
>
> Looking for suggestions (and a lesson or two!) This is my first attempt
> at SQL.

As Hugo pointed out, it is impossible to give very precise advice from
from the information you have posted. Assuming that there is an index
on (payment_method, property_id) on LEDGER_ENTRY, and that all other
tables have indexes on the columns you join on, I would expect the query
to perform well. Then again, there can be several reasons to why it does
not.

I analysed your query, and I think that I found one flaw. Here is a
rewritten version:

SELECT LE.entry_amount, LT.credit_card_exp_date, LE.entry_datetime,
LE.employee_id, LE.voucher_explanation, LE.card_reader_used_ind,
S.room_id, G.guest_lastname, G.guest_firstname, S.arrival_time,
S.departure_time, S.arrival_date, S.original_departure_date,
S.no_show_status, S.cancellation_date, F.house_acct_id,
F.group_code, LT.original_receipt_id
FROM mydb.dbo.LEDGER_ENTRY LE
JOIN mydb.dbo.LEDGER_TRANSACTON LT ON LE.trans_id = LT.trans_id
JOIN mydb.dbo.FOLIO F ON F.folio_id = LT.folio_id
LEFT JOIN (mydb.dbo.STAY_FOLIO SF
JOIN mydb.dbo.STAY S ON SF.stay_id = S.stay_id)
ON F.folio_id = SF.folio_id
LEFT JOIN mydb.dbo.GUEST G ON F.guest_id = G.guest_id
WHERE LE.payment_method='3737******6100'
AND LE.property_id='abc123'
ORDER BY LE.entry_datetime DESC

This alters the semantics of the query slightly, and I guess to the
good. Whether it affects performance, I don't know.

One potential problem is if the joins from FOLIO to STAY_FOLIO and GUEST
could hit multiple rows in the latter tables. In such case you get too
many rows back, which also could cause poor performance.


--
Erland Sommarskog, SQL Server MVP, esquel@xxxxxxxxxxxxx

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinfo/productdoc/2000/books.asp

.

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