MySQL - Foreign key on delete set null in not null field

Question

This is probably a trivial question, but I'm still a little clumsy when it comes to foreign key constraints so I wanted to make sure.

Let's say I have a table countries with the fields country_id (PK) and name , and a table cities with the fields city_id (PK), name and country_id (FK).

The foreign key cities.country_id has the constraint ON DELETE SET NULL . As I understand it, this means that if a record from countries is deleted, any records in cities that reference that deleted record's country_id will have its country_id field set to NULL.

What if, however, cities.country_id has the attribute NOT NULL ? Will this prevent the foreign key constraint from working properly? It would make sense that it does, but I just want to check.

Answer 1

If you set ON DELETE SET NULL to your foreign key then it won't allow you to set the field as NOT NULL .

So you won't be able to create or alter the table with column as NOT NULL and ON DELETE SET NULL on CountryId

When I run the below statements:

CREATE TABLE `country` (
  `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ;

CREATE TABLE `city` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `countryId` int(10) unsigned DEFAULT NOT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_country` (`countryId`),
  CONSTRAINT `FK_country` FOREIGN KEY (`countryId`) REFERENCES `country` (`id`) ON DELETE SET NULL ON UPDATE SET NULL
);

And I got the error in MySQL 5.5 is:

Schema Creation Failed: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NOT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_country` (`countryId`),
  CONSTRAINT `' at line 4: