Question

I have tables that look like this:

create table users (
    user_id uuid,
    user_name varchar(32)
);

create table messages (
    message_id uuid,
    from_id uuid references users(user_id),
    to_id uuid references users(user_id),
    the_message varchar(140),
    primary key(message_id)
);

If I do the following:

select * from messages

I will be given UUIDs. I want to display the users.user_name for the given uuid. This gets me close:

select
    users.user_name as from_id,
    users.user_name as to_id
from messages
    join users on users.user_id = messages.from_id
;

The result displays the same user for both the “from” and the “to” field.

How do I display the user_name for the from_id, and also the user_name for the to_id?

Answer 1

You need to JOIN the users table twice — once for the from user and once for the to user:

select
    messages.*,
    f.user_name as from_id,
    t.user_name as to_id
from messages
    join users f on f.user_id = messages.from_id
    join users t on t.user_id = messages.to_id
;