Question

I have a table in MySQL with multiple of reported events (primary key, date submitted, and text). Users can “like”, or “+1” the reports, similar to facebook/google. Right now, a user can “like” a single event infinite times. What’s the best way of arranging the database to check if he’s agreed to this even before?

I considered adding a column that can store a huge amount of text and adding a user to this text block if he agrees (comma seperated). I could then explode the text later and search it to see if his name is in there. However, I figured there must be a better way of going about this – any suggestions?

Answer 1

I’d use three tables: event, user and user_likes_event

CREATE TABLE `event` (
  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
  `submitted` DATE NULL ,
  `text` TEXT NULL ,
  PRIMARY KEY (`id`) )
ENGINE = InnoDB;

CREATE TABLE `user` (
  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
  `name` VARCHAR(45) NULL ,
  PRIMARY KEY (`id`) )
ENGINE = InnoDB;

CREATE TABLE `user_likes_event` (
  `event_id` INT UNSIGNED NOT NULL ,
  `user_id` INT UNSIGNED NOT NULL ,
  PRIMARY KEY (`event_id`, `user_id`) ,
  INDEX `fk_user_id` (`user_id` ASC) ,
  INDEX `fk_event_id` (`event_id` ASC) ,
  CONSTRAINT `fk_event_id`
    FOREIGN KEY (`event_id` )
    REFERENCES `event` (`id` )
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `fk_user_id`
    FOREIGN KEY (`user_id` )
    REFERENCES `user` (`id` )
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

Insert some values:

INSERT INTO `event` (submitted, text) VALUES
('2011-06-20', 'Event 1'),
('2011-06-21', 'Event 2'),
('2011-06-22', 'Event 3');

INSERT INTO `user` (name) VALUES
('Tom'), ('Dick'), ('Harry');

INSERT INTO `user_likes_event` (event_id, user_id) VALUES
(1,1), (1,2), (1,3),
(2,1), (2,2), (2,3),
(3,1), (3,2), (3,3);

Every user now likes every event once. If Tom attempts to like Event 1 again, a primary key constraint error is thrown:

INSERT INTO `user_likes_event` (event_id, user_id) VALUES (1,1);

ERROR 1062 (23000): Duplicate entry '1-1' for key 'PRIMARY'

If you don’t want errors thrown, you could try a conditional insert:

INSERT INTO `user_likes_event` (event_id, user_id)
SELECT 1,1 FROM DUAL
WHERE NOT EXISTS (
    SELECT event_id, user_id   FROM `user_likes_event`
    WHERE event_id = 1 AND user_id = 1
);

See this question for ideas on INSERT WHERE NOT EXISTS alternatives.

See the SELECT Syntax for more information on the DUAL table.