Question

I’ve got three tables:

CREATE TABLE credential_types (
    id serial PRIMARY KEY,
    name varchar(32) NOT NULL,
    ...
);

CREATE TABLE credentials (
    user_id integer REFERENCES users(id),
    credential integer REFERENCES credential_types(id),
    UNIQUE (user_id, credential)
);

CREATE TABLE users (
    id serial PRIMARY KEY,
    name varchar(64),
    username varchar(64),
    ...
);

Now, I want to fetch all users that exist, and I want to fetch the particular credential type they have. I tried this:

ribit=> SELECT u.id, u.name, u.username, ct.name AS cred \
         FROM users u, credential_types ct, credentials c \
         WHERE c.user_id=u.id AND ct.id=c.credential;
 id | name | username |  cred
----+------+----------+---------
  1 | foo  | bos      | Try-Out
(1 row)

Which is correct kind of output, but all the users that do not yet have a credential (null) are left out. I do not know which kind of JOIN I need for this operation. I think I need a LEFT JOIN, but I fail hard when I try to achieve what I want:

ribit=> SELECT u.id, u.name, u.username, ct.name \
        FROM users u, credential_types ct \
        LEFT JOIN credentials c ON ct.id=c.credential;

gives me a cross join with erronious data.

What I’m looking for are hints of how to write the query to produce the output I want.

Answer 1

The left join is not quite in the right place.

SELECT u.id, u.name, u.username, ct.name
FROM   users u
LEFT JOIN credentials c ON u.id = c.user_id
INNER JOIN credential_types ct ON c.credential = ct.id

I think this is closer to what you want.