Question

what is the best model for a system of tags? for example, a topic can have N tags, tags and N may be related to N threads, if I create a table, tags and posts a table and this table tags, I had the ID of the posts, and the tag, would be duplicating many records as another topic to add this tag, which is the best way to avoid this?

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@Quentin, the problem continues, it will repeat the records in the table post_tags

CREATE TABLE IF NOT EXISTS `posts` (
  `pid` bigint(22) NOT NULL AUTO_INCREMENT,
  `author` varchar(25) NOT NULL,
  `content` mediumtext NOT NULL,
  PRIMARY KEY (`pid`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;

INSERT INTO `posts` (`pid`, `author`, `content`) VALUES
(1, 'Andrey Knupp Vital', 'Hello World !');

CREATE TABLE IF NOT EXISTS `tagged` (
  `pid` bigint(22) NOT NULL,
  `tid` bigint(22) NOT NULL,
  PRIMARY KEY (`pid`,`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `tagged` (`pid`, `tid`) VALUES
(1, 1),
(1, 2);

CREATE TABLE IF NOT EXISTS `tags` (
  `tid` bigint(22) NOT NULL AUTO_INCREMENT,
  `tag` varchar(15) NOT NULL,
  PRIMARY KEY (`tid`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;

INSERT INTO `tags` (`tid`, `tag`) VALUES
(1, 'main'),
(2, 'hello');

Answer 1

Use a junction table

It will have at least two columns. “Posts” and “Tags” with the primary key being defined across the pair of them (if the database allows it, otherwise a third column for the primary key).