From: Jonathan Moules <jonathan-lists@lightpear.com>
To: pgsql-sql@postgresql.org
Subject: Not getting the expected results for a simple where not in
Date: Wed, 07 Jun 2017 13:20:13 +0100
Message-ID: <15c827e9305.ae02346553193.1322341002046903374@lightpear.com> (raw)
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-sql>
Hi List,
I'm a little confused by what seems like it should be a simple query and was hoping someone could explain what's going on.
Using PG 9.4.x
CREATE TABLE aaa.testing_nulls
(
str character varying(10),
status character varying(2)
)
Data:
"first";"aa"
"second";"aa"
"third";null
"fourth";"bb"
null;"aa"
null;"bb"
If I run:
select
str
from
aaa.testing_nulls
where
status in ('aa')
Against the table, I get the expected result:
"first"
"second"
null
But I want to get the items that don't have a value of 'aa'. Obviously in this case I can simply add "not" to the "where status in" but that's not suitable for my actual use-case (which is where this problem came to light). Instead, I'm nesting the original as a subquery:
select
*
from
aaa.testing_nulls
where
str not in
(
select
str
from
aaa.testing_nulls
where
status in ('aa')
)
Conceptually to me at least, this should work. I expect to get the values:
"third"
"fourth"
But instead when I run it I get 0 results.
It seems to relate to the nulls. If I change the above and add "and str is not null" into the subquery:
select
*
from
aaa.testing_nulls
where
str not in
(
select
str
from
aaa.testing_nulls
where
status in ('aa')
and str is not null
)
It now gives the expected results.
Why is this?
(I tested this in SQLite too, and get the same behaviour, so I guess it's a generic SQL thing I've never encountered before.)
Thanks,
Jonathan
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Subject: Re: Not getting the expected results for a simple where not in
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