MySQL Composite PK with Nullable FKs

Question

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.

The table I'm building has four foreign keys, two of which reference the same table. Here's the create statement for that table.

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;

The indexes I'm concerned with for this post are fk_reputation_log_user and fk_reputation_log_user1. Every reputation event has a giver, but only some have a receiver. I would like this FK to be nullable, but I don't know how to do that, or if it's even "allowed".

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.

If you need more detail, please state that in the comments. Thanks!

(Yes, this is for a reputation system not too dissimilar from what SO has)

Answer 1

CREATE  TABLE IF NOT EXISTS `abnr`.`reputation_event_log` (
  `id`                  INT NOT NULL AUTO_INCREMENT ,
  `reputation_event_id` INT NULL ,
  `giver_user_id`       INT NOT NULL , -- mandatory giver_user_id
  `receiver_user_id`    INT NULL ,     -- optional receiver_user_id
  . . .

Yes, you can have NULL in a column with a foreign key constraint declared on it. The NOT NULL constraint on a column is independent from any foreign key constraint(s) on that column.

A foreign key means that if the column has a non-NULL value, then that value must exist in the primary key of the table referenced by the foreign key constraint.

edit: As for your UNIQUE requirement, are you aware that you can declare a UNIQUE constraint on nullable columns. The column may contain NULLs (unlike the primary key constraint). This is standard SQL behavior, and is supported by MySQL.

  . . .
  PRIMARY KEY (`id`),
  CONSTRAINT UNIQUE (`giver_user_id`, `receiver_user_id`, 
                     `review_id`, `reputation_event_id`),
  . . .