how to make array_agg() work like group_concat() from mySQL

Question

So I have this table:

create table test (
   id integer, 
   rank integer,
   image varchar(30)
); 

Then some values:

i         


        

Answer 1

In PostgreSQL 8.4 you cannot explicitly order array_agg but you can work around it by ordering the rows passed into to the group/aggregate with a subquery:

SELECT id, array_to_string(array_agg(image), ',')
FROM (SELECT * FROM test ORDER BY id, rank) x
GROUP BY id;

In PostgreSQL 9.0 aggregate expressions can have an ORDER BY clause:

SELECT id, array_to_string(array_agg(image ORDER BY rank), ',')
FROM test
GROUP BY id;