It looks to me as though the "tracked.window=137" condition is suppressing output of
any "available" rows which do not have a target match in "tracked" - tracked.window
would = NULL in those cases.
--Greg Johnson
-----Original Message-----
From: George Eric R Contr AFSPC/CVYZ
[mailto:[EMAIL PROTECTED]]
Sent: Wednesday, October 03, 2001 11:50 AM
To: [EMAIL PROTECTED]
Subject: Left Join problem
I'm not getting the behavior I expect from this select.
SELECT available.target, tracked.sensor
FROM available LEFT JOIN tracked ON available.target=tracked.target
WHERE available.window=137 AND tracked.window=137 AND
available.sensor=8;
Now, I know for this window, that there are 25 records for sensor 8 in
the
available table:
SELECT count(*) FROM available WHERE window=137 AND sensor=8;
count(*)
25
Now in the tracked table, some of the targets that are available to
sensor 8
are actually tracked by another sensor, and some are not tracked at
all. I
expected the left join to show me which targets that are available to
target
8 are tracked by which sensor, with NULL values in the right column for
targets that are left untracked.
I get half of that. My result set has 20 records. It correctly shows
which
sensor tracked which target. It doesn't show the 4 untracked targets
at all?
Ideas?
Thanks
Eric
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