I am using array_to_json in combination with array_agg to format certain results in PostgreSQL as JSON. This works fine for queries in which I want to return the default value of a query (all columns, unmodified). But I'm stumped on how I could use array_agg to create a JSON object for a query in which I want to modify some of the output.
Here's an example. Let's say we have the following:
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 of course the following:
[{"user_id":1,"name":"Elise"},{"user_id":2,"name":"John"},{"user_id":3,"name":"User logged in from 10.0.0.9"}]
The following 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?
