Rails Active Record group query in SQLite vs Postgres - sql

Question

I am getting an error when running a query in SQLite vs Postgres with Active Record. There is an aggregate function error in Postgres but nothing in SQLite. My question is, what is SQLite doing here? Is it ignoring the group? Notice that GROUP BY "song" doesn't return an error.
SQLite output:
Playlist.where(user_id:user.id).group(:song)
Playlist Load (0.3ms) SELECT "playlists".* FROM "playlists" WHERE
"playlists"."user_id" = ? GROUP BY "song" [["user_id", 1]]
=> #<ActiveRecord::Relation [#<Playlist id: 2, user_id: 1, song_id: 1,
created_at: "2017-04-27 01:18:01", updated_at: "2017-04-27 01:18:01">]>
Playlist.where(user_id:user.id).group(:song_id)
Playlist Load (0.4ms) SELECT "playlists"."id", "playlists"."user_id",
"playlists"."song_id" FROM "playlists" WHERE "playlists"."user_id" = ?
GROUP BY "song" [["user_id", 1]]
=> #<ActiveRecord::Relation [#<Playlist id: 2, user_id: 1, song_id:
1, created_at: "2017-04-27 01:18:01">]>
Playlist.where(user_id:user.id).group(:id,:song_id)
Playlist Load (0.2ms) SELECT "playlists".* FROM "playlists" WHERE
"playlists"."user_id" = ? GROUP BY "playlists"."id",
"playlists"."song_id" [["user_id", 1]]
=> #<ActiveRecord::Relation [#<Playlist id: 1, user_id: 1, song_id: 1,
created_at: "2017-04-27 01:18:00", updated_at: "2017-04-27 01:18:00">,
#<Playlist id: 2, user_id: 1, song_id: 1, created_at: "2017-04-27
01:18:01", updated_at: "2017-04-27 01:18:01">]>
Playlist.where(user_id:user.id).group(:song).count
(0.2ms) SELECT COUNT(*) AS count_all, "playlists"."song_id" AS
playlists_song_id FROM "playlists" WHERE "playlists"."user_id" = ?
GROUP BY "playlists"."song_id" [["user_id", 1]]
Song Load (2.1ms) SELECT "songs".* FROM "songs"
WHERE "songs"."id" = ? LIMIT 1 [["id", 1]]
=> {#<Song id: 1, title: "A song", artist: "Artist", user_id: 1,
created_at: "2017-04-27 01:17:39">=>2}
Postgres:
Playlist.where(user_id:user.id).group(:song_id)
ActiveRecord::StatementInvalid: PG::GroupingError: ERROR: column
"playlists.id" must appear in the GROUP BY clause or be used in an
aggregate function
SELECT "playlists".* FROM "playlists" WHERE "playlists"."user_id" = $1
GROUP BY "playlists"."song_id"
Playlist.where(user_id:user.id).group(:id,:song_id)
Playlist Load (0.6ms) SELECT "playlists".* FROM "playlists" WHERE
"playlists"."user_id" = $1 GROUP BY "playlists"."id",
"playlists"."song_id" [["user_id", 1]]
=> #<ActiveRecord::Relation [#<Playlist id: 1, user_id: 1, song_id: 1,
created_at: "2017-04-27 01:25:34", updated_at: "2017-04-27 01:25:34">,
#<Playlist id: 2, user_id: 1, song_id: 1, created_at: "2017-04-27
01:25:36", updated_at: "2017-04-27 01:25:36">]>
Playlist.where(user_id:user.id).group(:song).count
(0.5ms) SELECT COUNT(*) AS count_all, "playlists"."song_id" AS
playlists_song_id FROM "playlists" WHERE "playlists"."user_id" = $1
GROUP BY "playlists"."song_id" [["user_id", 1]]
Song Load (0.3ms) SELECT "songs".* FROM "songs" WHERE "songs"."id" =
$1 LIMIT 1 [["id", 1]]
=> {#<Song id: 1, title: "A song", artist: "Artist", user_id: 1,
created_at: "2017-04-27 01:25:26", updated_at: "2017-04-27
01:25:26">=>2}

Answer 1

A common rule for group by: you may select only columns listed in group by and aggregate functions. This approach supported by sql standard and will work in any sql database.
If you are selecting columns not listed in a group by clause, for example select * … group by song_id, it may work in some databases and will not work in others.
If you specify selected columns in your code it will work:
Playlist.
where(user_id:user.id).
select("song_id").
group(:song_id)
You may read details about group by in PostgreSQL documentation: https://www.postgresql.org/docs/current/static/sql-select.html#SQL-GROUPBY