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Re: array_agg() NULL Handling

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 (view raw or flat)
Thread:
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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