# How to define alias in an ARRAY_AGG expression?

I'm trying to return pure JSON from a Postgres 9.2 table.

  SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
FROM (
SELECT
album,
max(release_year) AS release_year,
max(artwork_path) AS artwork_path,
MD5(concat(album,release_year,artist)) AS token,
ARRAY_AGG((media_files.position, media_files.token, media_files.title) ORDER BY media_files.position) as media_files
FROM media_files
INNER JOIN playlist_media_files ON playlist_media_files.media_file_id = media_files.id
WHERE playlist_media_files.playlist_id = 1
GROUP BY album, release_year, artist
ORDER BY artist, release_year
) as ALBUM_ROW


This query works fairly well, however on the line:

ARRAY_AGG((media_files.position, media_files.token) ORDER ...) as media_files


I would like to alias position and token attributes in the result set.

AS apparently it's not permitted here.

I would have written:

ARRAY_AGG((media_files.position AS xxx, media_files.token AS yyy) ORDER BY media_files.position) as media_files


But it's not working.

Postgres documentations says: "where aggregate_name is a previously defined aggregate (possibly qualified with a schema name), expression is any value expression that does not itself contain an aggregate expression or a window function call, and order_by_clause is a optional ORDER BY clause as described below."

http://www.postgresql.org/docs/9.3/static/sql-expressions.html#SYNTAX-AGGREGATES

Am I out of luck?

You are forming an ad-hoc row type (effectively an anonymous record) with this expression:

(media_files.position, media_files.token, media_files.title)


ARRAY_AGG((media_files.position, media_files.token, media_files.title)
ORDER BY media_files.position) AS media_files


Arrays types can only be built upon well-known types. Your option is to announce such a type to the system and cast the record to it before forming the array. Create a well-known composite type:

CREATE TYPE my_type AS (
position int    -- data type?
,token    text
,title    text
)


I am guessing data types for lack of information here. Fill in your actual types.

Creating a table has the same effect: It announces a well known composite type to the system indirectly, as well. For this reason, you can (ab-)use a temporary table to register a composite type for the duration of the session:

CREATE TEMP TABLE my_type AS (
position int    -- data type?
,token    text
,title    text
)


Either way, you can then cast your record:

ARRAY_AGG((media_files.position, media_files.token, media_files.title)::my_type
ORDER BY media_files.position) AS media_files

Then you can reference elements of the (now well-known) type by name:

SELECT media_files[1].position, media_files[1].token
FROM  (
...
,ARRAY_AGG((media_files.position, media_files.token, media_files.title)::my_type
ORDER BY media_files.position) AS media_files
...
FROM ....
GROUP BY ...
) sub;


Now, Postgres can use these names for building a JSON value. Voilá.

I don't have Postgres on my machine, and I really don't know it all that well, but it seems to me you could alias the desired columns through a subquery something like this:

SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
FROM (
SELECT
album,
max(release_year) AS release_year,
max(artwork_path) AS artwork_path,
MD5(concat(album,release_year,artist)) AS token,
ARRAY_AGG((mf.PositionAlias, mf.TokenAlias, mf.TitleAlias) ORDER BY mf.PositionAlias) as media_files
FROM (
SELECT
album,
max(release_year) AS release_year,
max(artwork_path) AS artwork_path,
MD5(concat(album,release_year,artist)) AS token,
position as PositionAlias,
token as TokenAlias,
title as TitleAlias
FROM media_files
INNER JOIN playlist_media_files ON playlist_media_files.media_file_id = media_files.id
WHERE playlist_media_files.playlist_id = 1
GROUP BY album, release_year, artist
ORDER BY artist, release_year
) AS mf
) as ALBUM_ROW;

• hello, I tried something similar but position is ambiguous, after I fixed it column "media_files.position" must appear in the GROUP BY clause or be used in an aggregate function. I'm currently experimenting with this but it seems over the top: pastie.org/private/kqksctmbgognytkeqovbya – John Smith Nov 13 '13 at 7:42