Hi Carla!
Well, maybe I too simplified my production code and now I can't see
something very simple, BUT I'm pretty sure that there isn't any original
column value2 in sub4 except that I created with COALESCE:
Meanwhile, I want to note, that I made a little mistake in presented
example: instead of "wrong" full example I wrote a "correct" one (but I'm
sure you understood this because of my further explanation of "workaround"
=))
My mistake =(( don't kill me - I spend too much time with this piece of code
today...
but just in case here is "wrong" (difference is in that "SELECT sub3.key3,
sub4.value2 FROM" ):
SELECT * FROM
(
SELECT 1 as key1
) sub1
LEFT JOIN
(
SELECT sub3.key3, sub4.value2 FROM
(
SELECT 1 as key3
) sub3
LEFT JOIN
(
SELECT sub5.key5, COALESCE(sub6.value1, 1) as
value2
FROM
(
SELECT 1 as key5
) sub5
LEFT JOIN
(
SELECT 1 as key6, value1
FROM
(
SELECT
NULL::integer as value1
) sub7
WHERE false
) sub6 ON false
)
sub4 ON sub4.key5=sub3.key3
)
sub2 ON sub1.key1 = sub2.key3
best regards,
alex
From: [email protected] [mailto:[email protected]]
On Behalf Of Carla
Sent: Monday, August 08, 2011 10:03 PM
To: ai
Cc: [email protected]
Subject: Re: [SQL] Problem with nested left-joins and coalesce
Hi Alex!
When you wrote "COALESCE(sub6.value1, 1) as value2", you created a column
"value2" that is different of the original column "sub4.value2".
Try running "SELECT sub3.key3, sub4.value2, value2 FROM ..." and you'll get
the result:
key1;key3;value2;value2
1;1;null;1
It happens because the first column "value2" (i.e. "sub4.value2") doesn't
have COALESCE on it.
2011/8/8 ai
Hi!
I have strange issue with nested left-joins in postgresql...
It's hard to explain, but easy to show =)
here we are:
SELECT * FROM
(
SELECT 1 as key1
) sub1
LEFT JOIN
(
SELECT sub3.key3, value2 FROM
(
SELECT 1 as key3
) sub3
LEFT JOIN
(
SELECT sub5.key5, COALESCE(sub6.value1, 1) as
value2
FROM
(
SELECT 1 as key5
) sub5
LEFT JOIN
(
SELECT 1 as key6, value1
FROM
(
SELECT
NULL::integer as value1
) sub7
WHERE false
) sub6 ON false
)
sub4 ON sub4.key5=sub3.key3
)
sub2 ON sub1.key1 = sub2.key3
The result of this query:
key1;key3;value2
1;1;NULL
And this is the problem - value2 can't be NULL because of COALESCE in sub4
(at least I think that it can't be =))
Anyway if we'll change
SELECT sub3.key3, sub4.value2 FROM
with
SELECT sub3.key3, value2 FROM
we will got correct result:
key1;key3;value2
1;1;1
Is there something wrong with my mind&hands? or is it a bug?
Thanks in advance!
Kind regards
Alex
