Postgres - aggregate two columns into one item

Question

I would like to aggregate two columns into one \"array\" when grouping.

Assume a table like so:

friends_map:
=================================
user_i         


        

Answer 1

You can concatenate the values together prior to feeding them into the array_agg() function:

SELECT user_id, array_agg('[' || friend_id || ',' || confirmed || ']') as friends
FROM friends_map
WHERE user_id = 1
GROUP BY user_id

Demo: SQL Fiddle

Answer 2

In Postgres 9.5 you can obtain array of arrays of text:

SELECT user_id, array_agg(array[friend_id::text, confirmed::text])
FROM friend_map
WHERE user_id = 1
GROUP BY user_id;

 user_id |           array_agg            
---------+--------------------------------
       1 | {{2,true},{3,false},{4,false}}
(1 row)

or array of arrays of int:

SELECT user_id, array_agg(array[friend_id, confirmed::int])
FROM friend_map
WHERE user_id = 1
GROUP BY user_id;

 user_id |      array_agg      
---------+---------------------
       1 | {{2,1},{3,0},{4,0}}
(1 row) 

Answer 3

You could avoid the ugliness of the multidimentional array and use some json which supports mixed datatypes:

SELECT user_id, json_agg(json_build_array(friend_id, confirmed)) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id

Or use some key : value pairs since json allows that, so your output will be more semantic if you like:

SELECT user_id, json_agg(json_build_object(
        'friend_id', friend_id, 
        'confirmed', confirmed
    )) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id;

Answer 4

You can use: array[]

Just do that:

SELECT user_id, array[friend_id, confirmed]
FROM friend_map
WHERE user_id = 1
GROUP BY 1;

Answer 5

SELECT user_id, array_agg((friend_id, confirmed)) as friends
FROM friend_map
WHERE user_id = 1
GROUP BY user_id

user_id |           array_agg            
--------+--------------------------------
      1 | {"(2,true)","(3,false)","(4,false)"}