Check out example codes for "mysql add foreign key". It will help you in understanding the concepts better.

Code Example 1

ALTER TABLE Orders

ADD CONSTRAINT FK_PersonOrder

FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID);

Code Example 2

CREATE TABLE parent (
    id INT NOT NULL,
    PRIMARY KEY (id)
) ENGINE=INNODB;

CREATE TABLE child (
    id INT,
    parent_id INT,
    INDEX par_ind (parent_id),
    FOREIGN KEY (parent_id)
        REFERENCES parent(id)
        ON DELETE CASCADE
) ENGINE=INNODB;

Code Example 3

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

Code Example 4

-- 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;

Code Example 5

ALTER TABLE table_name
DROP CONSTRAINT fk_name;

Code Example 6

SET foreign_key_checks = 0;
UPDATE languages SET id='xyz' WHERE id='abc';
UPDATE categories_languages SET language_id='xyz' WHERE language_id='abc';
SET foreign_key_checks = 1;

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