From: | "David E(dot) Wheeler" <david(at)kineticode(dot)com> |
---|---|
To: | Thom Brown <thom(at)linux(dot)com> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: array_agg() NULL Handling |
Date: | 2010-09-01 07:03:41 |
Message-ID: | 63C3C7B1-5C2A-4EC4-A4F6-856FD2949434@kineticode.com |
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Lists: | pgsql-hackers |
On Aug 31, 2010, at 11:56 PM, Thom Brown wrote:
>>> The first form of aggregate expression invokes the aggregate across all input rows for which the given expression(s) yield non-null values. (Actually, it is up to the aggregate function whether to ignore null values or not — but all the standard ones do.)
>>
>> -- http://developer.postgresql.org/pgdocs/postgres/sql-expressions.html#SYNTAX-AGGREGATES
>>
>> That, however, is not true of array_agg():
>>
>> try=# CREATE TABLE foo(id int);
>> CREATE TABLE
>> try=# INSERT INTO foo values(1), (2), (NULL), (3);
>> INSERT 0 4
>> try=# select array_agg(id) from foo;
>> array_agg
>> ──────────────
>> {1,2,NULL,3}
>> (1 row)
>>
>> So are the docs right, or is array_agg() right?
>
> I think it might be both. array_agg doesn't return NULL, it returns
> an array which contains NULL.
No, string_agg() doesn't work this way, for example:
select string_agg(id::text, ',') from foo;
string_agg
────────────
1,2,3
(1 row)
Note that it's not:
select string_agg(id::text, ',') from foo;
string_agg
────────────
1,2,,3
(1 row)
Best,
David
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