MySQL 8.0

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CREATE TABLE posts(
  post_id int unsigned NOT NULL AUTO_INCREMENT,
  content varchar(200) NOT NULL,
  PRIMARY KEY (post_id)
);
INSERT INTO posts VALUES 
(1, 'Post 1'),
(2, 'Post 2');

CREATE TABLE hashtags(
  hashtag_id int unsigned NOT NULL AUTO_INCREMENT,
  hashtag varchar(40) NOT NULL,
  PRIMARY KEY (hashtag_id)
);
INSERT INTO hashtags VALUES
(1, 'Hashtag 1'),
(2, 'Hashtag 2'),
(3, 'Hashtag 3'),
(4, 'Hashtag 4'),
(5, 'Hashtag 5'),
(6, 'Hashtag 6');

CREATE TABLE posts_hashtags_relation(
  post_id int unsigned NOT NULL,
  hashtag_id int unsigned NOT NULL,
  PRIMARY KEY (post_id, hashtag_id)
);
INSERT INTO posts_hashtags_relation VALUES
(1,1), (1,2), (1,3),
(2,3), (2,4), (2,5);

SELECT * FROM posts;
SELECT * FROM hashtags;
SELECT * FROM posts_hashtags_relation;

Records: 2  Duplicates: 0  Warnings: 0



Records: 6  Duplicates: 0  Warnings: 0



Records: 6  Duplicates: 0  Warnings: 0




post_id
content


1
Post 1

2
Post 2




hashtag_id
hashtag


1
Hashtag 1

2
Hashtag 2

3
Hashtag 3

4
Hashtag 4

5
Hashtag 5

6
Hashtag 6




post_id
hashtag_id


1
1

1
2

1
3

2
3

2
4

2
5


CREATE PROCEDURE save_post_and_tags (post_content VARCHAR(200), hashtag_ids TEXT)
BEGIN
    DECLARE post_id INT UNSIGNED;
    DECLARE tag_id INT UNSIGNED;
    INSERT INTO posts VALUES (DEFAULT, post_content);
    SET post_id := LAST_INSERT_ID();
    SET hashtag_ids := concat(hashtag_ids, ',');
    REPEAT
        SET tag_id := 0 + SUBSTRING_INDEX(hashtag_ids, ',', 1);
        SET hashtag_ids := TRIM(SUBSTRING(hashtag_ids FROM 1 + LOCATE(',', hashtag_ids)));
        INSERT INTO posts_hashtags_relation VALUES (post_id, tag_id);
    UNTIL hashtag_ids = '' END REPEAT;
END
CALL save_post_and_tags ('Post No. 3', ' 1, 3 ,  5   ');

SELECT * FROM posts;
SELECT * FROM hashtags;
SELECT * FROM posts_hashtags_relation;


post_id
content


1
Post 1

2
Post 2

3
Post No. 3




hashtag_id
hashtag


1
Hashtag 1

2
Hashtag 2

3
Hashtag 3

4
Hashtag 4

5
Hashtag 5

6
Hashtag 6




post_id
hashtag_id


1
1

1
2

1
3

2
3

2
4

2
5

3
1

3
3

3
5
hashtag_id	hashtag
1	Hashtag 1
2	Hashtag 2
3	Hashtag 3
4	Hashtag 4
5	Hashtag 5
6	Hashtag 6