constraint mysql foreign key code example

Example 1: mysql change foreign key

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

Example 2: 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 3: how to connect database foreign key in mysql

CREATE TABLE products(
    productId INT AUTO_INCREMENT PRIMARY KEY,
    productName varchar(100) not null,
    categoryId INT NOT NULL,
    CONSTRAINT fk_category
    FOREIGN KEY (categoryId) 
    REFERENCES categories(categoryId)
        ON UPDATE CASCADE
        ON DELETE CASCADE
) ENGINE=INNODB;

Example 4: alter table add column forigen key mysql

ALTER TABLE tryholpz_demo07.core_modules 
ADD COLUMN belongs_to_role INT, 
ADD FOREIGN KEY core_modules(belongs_to_role) REFERENCES role_specific_modules_info(id) ON DELETE CASCADE