Question

I am using array_to_json in combination with array_agg to format certain results in PostgreSQL as JSON. This works fine for queries when I want to return the default value of a query (all columns, unmodified). But I’m stumped how I could use array_agg to create a JSON object for a query where I want to modify some of the output.

Here’s an example:

CREATE TABLE temp_user ( 
   user_id  serial PRIMARY KEY,
   real_name text
);
CREATE TABLE temp_user_ip (
   user_id  integer,
   ip_address text
);
INSERT INTO temp_user (user_id, real_name) VALUES (1, 'Elise'),  (2, 'John'), (3, NULL);
INSERT INTO temp_user_ip (user_id, ip_address) VALUES (1, '10.0.0.4'),  (2, '10.0.0.7'), (3, '10.0.0.9');

The following query works fine:

# SELECT array_to_json(array_agg(temp_user)) as users from temp_user;
                                            users                                                
-----------------------------------------------------------------------------------------------------
 [{"user_id":1,"real_name":"Elise"},{"user_id":2,"real_name":"John"},{"user_id":3,"real_name":null}]

But let’s say that I don’t like the null value appearing for user 3. I’d rather see the string “User logged in from $ip” instead.

I can do this:

# SELECT user_id, (CASE WHEN real_name IS NULL THEN (select 'User logged in from ' || ip_address FROM temp_user_ip WHERE user_id = temp_user.user_id) ELSE real_name END) as name from temp_user;

And I get the following results:

 user_id |             name             
---------+------------------------------
       1 | Elise
       2 | John
       3 | User logged in from 10.0.0.9

Which is great. But I can’t figure out how to manipulate this data into JSON format like the first example.

The desired output is:

[{"user_id":1,"name":"Elise"},{"user_id":2,"name":"John"},{"user_id":3,"name":"User logged in from 10.0.0.9"}]

This doesn’t work:

# select array_to_json(array_agg ( (SELECT user_id, (CASE WHEN real_name IS NULL THEN (select 'User logged in from ' || ip_address FROM temp_user_ip WHERE user_id = temp_user.user_id) ELSE real_name END) as name from temp_user)));
ERROR:  subquery must return only one column

I can’t figure out any way to get the data into a format that array_agg accepts. I even tried creating a custom type which matched the format of temp_user and trying to array_agg calls to the type constructor, which returned the same error. The error doesn’t make sense to me – if the subquery is aggregated, then it shouldn’t matter if it returns more than one column. Any advice?

Answer 1

You can separate the aggregate call from the subquery and use the row constructor for generating the compound data:

SELECT 
    array_to_json(array_agg(row(t.*))) AS users 
FROM 
    (
        SELECT user_id, 
            CASE 
                WHEN real_name IS NULL 
                THEN (
                    SELECT 'User logged in from ' || ip_address 
                    FROM temp_user_ip 
                    WHERE user_id = temp_user.user_id
                ) ELSE real_name 
            END AS name 
        FROM temp_user
    ) t
;

You can also check this on SQLFiddle.