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POMySQL Composite PK with Nullable FKs
 

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    <p>First off, let me preface this question by stating that I'm really a pretty terrible data modeler. I know only enough to be dangerous.</p> <p>The table I'm building has four foreign keys, two of which reference the same table. Here's the create statement for that table.</p> <pre><code>CREATE TABLE IF NOT EXISTS `abnr`.`reputation_event_log` ( `id` INT NOT NULL AUTO_INCREMENT , `reputation_event_id` INT NULL , `giver_user_id` INT NULL , `receiver_user_id` INT NULL , `review_id` INT NULL , `giver_point_value` SMALLINT NULL DEFAULT 0 , `receiver_point_value` SMALLINT NULL DEFAULT 0 , `created_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP , PRIMARY KEY (`id`) , INDEX `fk_reputation_log_user` (`giver_user_id` ASC) , INDEX `fk_reputation_log_user1` (`receiver_user_id` ASC) , INDEX `fk_reputation_log_review` (`review_id` ASC) , INDEX `fk_reputation_log_reputation_event` (`reputation_event_id` ASC) , CONSTRAINT `fk_reputation_log_user` FOREIGN KEY (`giver_user_id` ) REFERENCES `abnr`.`user` (`id` ) ON DELETE NO ACTION ON UPDATE NO ACTION, CONSTRAINT `fk_reputation_log_user1` FOREIGN KEY (`receiver_user_id` ) REFERENCES `abnr`.`user` (`id` ) ON DELETE NO ACTION ON UPDATE NO ACTION, CONSTRAINT `fk_reputation_log_review` FOREIGN KEY (`review_id` ) REFERENCES `abnr`.`review` (`id` ) ON DELETE NO ACTION ON UPDATE NO ACTION, CONSTRAINT `fk_reputation_log_reputation_event` FOREIGN KEY (`reputation_event_id` ) REFERENCES `abnr`.`reputation_event` (`id` ) ON DELETE NO ACTION ON UPDATE NO ACTION) ENGINE = InnoDB DEFAULT CHARACTER SET = utf8 COLLATE = utf8_general_ci; </code></pre> <p>The indexes I'm concerned with for this post are <code>fk_reputation_log_user</code> and <code>fk_reputation_log_user1</code>. Every reputation event has a giver, but only some have a receiver. I would <em>like</em> this FK to be nullable, but I don't know how to do that, or if it's even "allowed".</p> <p>I also toyed with the idea of making all FK columns part of the primary key to have database-level protection against duplicate log entries - but that won't work since PK columns have to be NOT NULL.</p> <p>If you need more detail, please state that in the comments. Thanks!</p> <p>(Yes, this is for a reputation system not too dissimilar from what SO has)</p>
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