alter table add column with foreign key code example

Example 1: mysql add foreign key

-- On Create
CREATE TABLE tableName (
    ID INT,
    SomeEntityID INT,
    PRIMARY KEY (ID),
    FOREIGN KEY (SomeEntityID)
        REFERENCES SomeEntityTable(ID)
        ON DELETE CASCADE
);

-- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
  
 -- Add FK with a specific name
 -- On Alter, if the column already exists but has no FK
ALTER TABLE
  tableName
ADD CONSTRAINT fk_name
  FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;

Example 2: add constraint fk

ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

Example 3: add new column in sql with foreign key

ALTER TABLE `fb_banners`
ADD `city_id` BIGINT(20) UNSIGNED NULL DEFAULT NULL AFTER `BannerID`,
ADD INDEX `Foreign_city_id_banners` (`city_id`) USING BTREE,
ADD CONSTRAINT `Foreign_city_id_banners` FOREIGN KEY (`city_id`) REFERENCES `foodsafari`.`fb_cities` (`id`) ON UPDATE CASCADE ON DELETE CASCADE;