Re: Problem with nested left-joins and coalesce

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(at)postgresql(dot)org [mailto:pgsql-sql-owner(at)postgresql(dot)org]
On Behalf Of Carla
Sent: Monday, August 08, 2011 10:03 PM
To: ai
Cc: pgsql-sql(at)postgresql(dot)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(at)mail(dot)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