Re: Problem with nested left-joins and coalesce - Mailing list pgsql-sql
| From | ai |
|---|---|
| Subject | Re: Problem with nested left-joins and coalesce |
| Date | |
| Msg-id | 01bd01cc55e0$bd6a3030$383e9090$@mail.ru Whole thread Raw |
| In response to | Re: Problem with nested left-joins and coalesce (Carla <cgourofino@hotmail.com>) |
| Responses |
Re: Problem with nested left-joins and coalesce
|
| List | pgsql-sql |
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: pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Carla
Sent: Monday, August 08, 2011 10:03 PM
To: ai
Cc: pgsql-sql@postgresql.org
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 <listar@mail.ru>
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