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

问题

So I have this table:

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

Then some values:

id | rank | image   
---+------+-------  
 1 |    2 | bbb  
 1 |    3 | ccc  
 1 |    1 | aaa  
 2 |    3 | c  
 2 |    1 | a  
 2 |    2 | b  

I want to group them by id and concatenate the image name in the order given by rank. In mySQL I can do this:

  select id, 
         group_concat( image order by rank asc separator ',' ) 
    from test 
group by id;

And the output would be:

1 aaa,bbb,ccc
2 a,b,c

Is there a way I can have this in postgresql?

If I try to use array_agg() the names will not show in the correct order and apparently I was not able to find a way to sort them. (I was using postgres 8.4 )

回答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;

来源：https://stackoverflow.com/questions/4054559/how-to-make-array-agg-work-like-group-concat-from-mysql