我如何避免在mysql中有兩個外鍵的空值？

我得到了一張表user，一張表assignment和一張表candidation（帶有一個主要的，而不是自然的），它有兩個外鍵。它們都是1：nc，表示不一定有用戶進行候選（用戶刪除）或分配（分配已刪除）。我如何避免在mysql中有兩個外鍵的空值？

現在我怎麼能避免在中有NULL這兩個外鍵？

USER：

-- Table `Ressource-Management`.`user` 
-- ----------------------------------------------------- 
CREATE TABLE IF NOT EXISTS `Ressource-Management`.`user` (
    `iduser` INT UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '', 
    `name_first` VARCHAR(50) NOT NULL COMMENT '', 
    `name_last` VARCHAR(50) NOT NULL COMMENT '', 
    `attendance` INT UNSIGNED NOT NULL COMMENT '', 
    `can_own` TINYINT(1) NOT NULL DEFAULT 0 COMMENT '', 
    PRIMARY KEY (`iduser`) COMMENT '') 
ENGINE = InnoDB;

作業：

-- ----------------------------------------------------- 
-- Table `Ressource-Management`.`assignment` 
-- ----------------------------------------------------- 
CREATE TABLE IF NOT EXISTS `Ressource-Management`.`assignment` (
    `idassignment` INT UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '', 
    `owner` INT UNSIGNED NOT NULL COMMENT '', 
    `budget` INT UNSIGNED NOT NULL COMMENT '', 
    `priority` TINYINT UNSIGNED NOT NULL DEFAULT 0 COMMENT '', 
    `date_to` DATETIME NULL COMMENT '', 
    PRIMARY KEY (`idassignment`) COMMENT '', 
    INDEX `fk_assignment_user_idx` (`owner` ASC) COMMENT '', 
    CONSTRAINT `fk_assignment_user` 
    FOREIGN KEY (`owner`) 
    REFERENCES `Ressource-Management`.`user` (`iduser`) 
    ON DELETE RESTRICT 
    ON UPDATE CASCADE) 
ENGINE = InnoDB;

CANDIDATION：

-- ----------------------------------------------------- 
-- Table `Ressource-Management`.`candidation` 
-- ----------------------------------------------------- 
CREATE TABLE IF NOT EXISTS `Ressource-Management`.`candidation` (
    `idcandidation` INT UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '', 
    `user_iduser` INT UNSIGNED NULL COMMENT '', 
    `assignment_idassignment` INT UNSIGNED NULL COMMENT '', 
    `date_from` DATETIME NOT NULL COMMENT '', 
    `date_to` DATETIME NOT NULL COMMENT '', 
    `status` LONGBLOB NULL DEFAULT offen COMMENT '', 
    `message` VARCHAR(4096) NULL COMMENT '', 
    PRIMARY KEY (`idcandidation`) COMMENT '', 
    INDEX `fk_user_has_assignment_assignment1_idx` (`assignment_idassignment` ASC) COMMENT '', 
    INDEX `fk_user_has_assignment_user1_idx` (`user_iduser` ASC) COMMENT '', 
    UNIQUE INDEX `user_assignment_UNIQUE` (`user_iduser` ASC, `assignment_idassignment` ASC) COMMENT '', 
    CONSTRAINT `fk_user_has_assignment_user1` 
    FOREIGN KEY (`user_iduser`) 
    REFERENCES `Ressource-Management`.`user` (`iduser`) 
    ON DELETE SET NULL 
    ON UPDATE CASCADE, 
    CONSTRAINT `fk_user_has_assignment_assignment1` 
    FOREIGN KEY (`assignment_idassignment`) 
    REFERENCES `Ressource-Management`.`assignment` (`idassignment`) 
    ON DELETE SET NULL 
    ON UPDATE CASCADE) 
ENGINE = InnoDB;

來源

2015-09-04 inetphantom

你可以在外部表'tableA'的一個創建觸發器和丟棄刪除，如果已經在其他創建觸發器一個'tableB'是'NULL'。 –

@JuanCarlosOropeza需要兩張桌子才能生效。 _But ..我不喜歡觸發器.._ – inetphantom