PostgreSQL query with FULL OUTER JOIN and subquery using column of GROUP BY expression ERROR: subquery uses ungrouped column from outer query

Hello,

I have 3 tables : t1, t2 and t3
t1(posx integer, posy integer, dat timestamp, val_grid numeric)
with Primary Key=(posx, posy, dat)
t2(id integer, x integer, y integer)
with Primary Key=(id)
t3(id integer, dat timestamp, c1 numeric)
with Primary Key=(id, dat)

My (very simplified) query that doesn't work :
SELECT
id, to_char(dat, 'YYYYMM'), CASE FLOOR((TO_NUMBER(TO_CHAR(dat,'DD'),'99')-1)/10) WHEN 0 THEN 1 WHEN 1 THEN 2 ELSE 3 END::smallint,
avg(val_grid), sum(c1),
(SELECT col1 FROM another_table WHERE another_table.np=id LIMIT 1)
FROM
(SELECT * FROM t1 ) AS t1
JOIN t2 ON (t1.posx=t2.x AND t1.posy=t2.y)
FULL OUTER JOIN (SELECT * FROM t3 ) t3 USING(id, dat)
--WHERE ...
GROUP BY id, to_char(dat, 'YYYYMM'), CASE FLOOR((TO_NUMBER(TO_CHAR(dat,'DD'),'99')-1)/10) WHEN 0 THEN 1 WHEN 1 THEN 2 ELSE 3 END::smallint
;
ERROR: subquery uses ungrouped column "t2.id" from outer query

-> The problem is on the third line (the subquery) :
(SELECT col1 FROM another_table WHERE another_table.np=id LIMIT 1)

However, If I replace " FULL OUTER " by " LEFT OUTER " or by " RIGHT OUTER ", it works .

Could you please tell me how to correct the query, to fix this problem?

Regards
----- Météo-France -----
PALAYRET Jacques
DCSC/GDC
jacques(dot)palayret(at)meteo(dot)fr
Fixe : +33 561078319