A demonstration of recursive query in SQL

recurstive_cte.sql

CREATE TABLE posts
(
    id          bigserial PRIMARY KEY,
    content     text NOT NULL,
    reply_to_id bigint REFERENCES posts (id)
);

TRUNCATE posts RESTART IDENTITY; -- this deletes all rows and restarts the auto-increment ID

/*
Let's create nested data like this:
-> R
    -> R-O
        -> R-O-A
            -> R-O-A-D
        -> R-O-D
    -> R-A
        -> R-A-P
-> S
    -> H
*/

INSERT INTO posts(content)
VALUES ('R');

INSERT INTO posts(content, reply_to_id)
VALUES ('RO', 1),
       ('RA', 1);

INSERT INTO posts(content, reply_to_id)
VALUES ('ROA', 2),
       ('RAP', 3);

INSERT INTO posts(content, reply_to_id)
VALUES ('ROAD', 4);

INSERT INTO posts(content, reply_to_id)
VALUES ('ROD', 2);

INSERT INTO posts(content)
VALUES ('S');

INSERT INTO posts(content, reply_to_id)
VALUES ('SH', 7);

SELECT *
FROM posts;


-- select all children for 'R'
WITH RECURSIVE leaves(id, content, reply_to_id) AS
                   (SELECT id, content, reply_to_id
                    FROM posts
                    WHERE content = 'R'

                    UNION ALL

                    SELECT p.id, p.content, p.reply_to_id
                    FROM leaves l
                             INNER JOIN posts p ON l.id = p.reply_to_id)
SELECT *
FROM leaves;

-- select all children for 'S'
WITH RECURSIVE leaves(id, content, reply_to_id) AS
                   (SELECT id, content, reply_to_id
                    FROM posts
                    WHERE content = 'S'

                    UNION ALL

                    SELECT p.id, p.content, p.reply_to_id
                    FROM leaves l
                             INNER JOIN posts p ON l.id = p.reply_to_id)
SELECT *
FROM leaves;

-- select all children for 'RO'
WITH RECURSIVE leaves(id, content, reply_to_id) AS
                   (SELECT id, content, reply_to_id
                    FROM posts
                    WHERE content = 'RO'

                    UNION ALL

                    SELECT p.id, p.content, p.reply_to_id
                    FROM leaves l
                             INNER JOIN posts p ON l.id = p.reply_to_id)
SELECT *
FROM leaves;