数据（我需要稍作更改，因为不应将其分配给序列号）：
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 postSELECt 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_ObamaFROM posts p ;
结果：
id | title | liked_by_obama ----+-------------+---------------- 1 | First Post | t 2 | Second Post | f 3 | Third Post | f(3 rows)
