Foreign Key equal Null - mysql

Question

I have tabel users:
CREATE TABLE IF NOT EXISTS `ePrzychodnia`.`users` (
`id` INT NOT NULL AUTO_INCREMENT ,
`firstName` VARCHAR(45) NOT NULL ,
`lastName` VARCHAR(45) NOT NULL ,
`personalId` VARCHAR(11) NOT NULL ,
`password` VARCHAR(45) NOT NULL ,
`city` VARCHAR(45) NULL ,
`address` VARCHAR(45) NULL ,
`email` VARCHAR(45) NULL ,
`phone` VARCHAR(45) NULL ,
`specialization` VARCHAR(45) NULL ,
`role` VARCHAR(45) NOT NULL ,
`active` TINYINT(1) NOT NULL ,
PRIMARY KEY (`id`) ,
UNIQUE INDEX `personalId_UNIQUE` (`personalId` ASC) )
and tabel visits. Tabel visits have two foreign key to the users table
CREATE TABLE IF NOT EXISTS `ePrzychodnia`.`visits` (
`id` INT NULL AUTO_INCREMENT ,
`doctorId` INT NOT NULL ,
`patientId` INT NULL,
`dateVisit` DATE NOT NULL ,
`hourVisit` DATE NOT NULL ,
PRIMARY KEY (`id`, `doctorId`, `patientId`) ,
INDEX `fk_visits_Users1_idx` (`doctorId` ASC) ,
INDEX `fk_visits_Users2_idx` (`patientId` ASC) ,
CONSTRAINT `fk_visits_Users1`
FOREIGN KEY (`doctorId` )
REFERENCES `ePrzychodnia`.`users` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_visits_Users2`
FOREIGN KEY (`patientId` )
REFERENCES `ePrzychodnia`.`users` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
The second foreign key I set that can be null, because I care about adding an entry to the table visits where it will be empty.
However, when you try to add I get an error
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (eprzychodnia.visits, CONSTRAINT fk_visits_Users2 FOREIGN KEY (patientId) REFERENCES users (id) ON DELETE NO ACTION ON UPDATE NO ACTION)
What can I do to make the key could be null?

Answer 1

In your relation setup change the action on update and delete

Answer 2

The problem is not your foreign key, it's your primary key. You can't have nullable fields in a primary key. The id field by itself might be a better choice.
I'm not a fan of nullable foreign keys. I would add a record to your users table that equates to "not an actual person" and use it as a default. By the way, the hourvisit field might be redundant.

Answer 3

Dana Bruck I do not know if I understand your proposals. But it seems to me that it's something like this:
Table visits
CREATE TABLE IF NOT EXISTS `ePrzychodnia`.`visits` (
`id` INT NULL AUTO_INCREMENT ,
`doctorId` INT NOT NULL ,
`patientId` INT NULL,
`dateVisit` DATE NOT NULL ,
`hourVisit` DATE NOT NULL ,
PRIMARY KEY (`id`, `doctorId`, `patientId`))
Table without foreign keys. The fields not related to the table users, but I set these fields when I need and this field may be null.

Answer 4

If the id in visits is auto_increment, why you create your PK with 3 columns ??
Just id is enough. You can create a unique index for doctor and patience ID.
Try this model:
CREATE TABLE IF NOT EXISTS `ePrzychodnia`.`visits` (
`id` INT NOT NULL AUTO_INCREMENT ,
`doctorId` INT NOT NULL ,
`patientId` INT NULL,
`dateVisit` DATE NOT NULL ,
`hourVisit` DATE NOT NULL ,
PRIMARY KEY (`id`) ,
UNIQUE INDEX `unique_doctor_pacient` (`doctorId`, `patientId`),
INDEX `fk_visits_Users1_idx` (`doctorId` ASC) ,
INDEX `fk_visits_Users2_idx` (`patientId` ASC) ,
CONSTRAINT `fk_visits_Users1`
FOREIGN KEY (`doctorId` )
REFERENCES `ePrzychodnia`.`users` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_visits_Users2`
FOREIGN KEY (`patientId` )
REFERENCES `ePrzychodnia`.`users` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)