Daten (musste ich etwas ändern, da man Serien nicht zuordnen sollte):
CREATE TABLE posts (
id serial,
title varchar
);
CREATE TABLE users (
id serial,
name varchar
);
CREATE TABLE likes (
id serial,
user_id int,
post_id int
);
INSERT INTO posts (title) VALUES ('First Post');
INSERT INTO posts (title) VALUES ('Second Post');
INSERT INTO posts (title) VALUES ('Third Post');
INSERT INTO users (name) VALUES ('Obama');
INSERT INTO users (name) VALUES ('Trump');
INSERT INTO likes (user_id, post_id) VALUES (1, 1);
INSERT INTO likes (user_id, post_id) VALUES (2, 1);
INSERT INTO likes (user_id, post_id) VALUES (2, 2);
-- I want to retrieve a list of ALL of the posts and whether or not a specific user has liked that post
SELECT id, title
, EXISTS(
--EXISTS() yields a boolean value
SELECT *
FROM likes lk
JOIN users u ON u.id = lk.user_id AND lk.post_id=p.id
WHERE u.name ='Obama'
) AS liked_by_Obama
FROM posts p
;
Ergebnisse:
id | title | liked_by_obama
----+-------------+----------------
1 | First Post | t
2 | Second Post | f
3 | Third Post | f
(3 rows)
