how to set an attribute as foreign key in sql 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: MySQL Foreign Key

Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement:

[CONSTRAINT constraint_name]
FOREIGN KEY [foreign_key_name] (column_name, ...)
REFERENCES parent_table(colunm_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]
In this syntax:

First, specify the name of foreign key constraint that you want to create after the CONSTRAINT keyword. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint.

Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords. The foreign key name is also optional and is generated automatically if you skip it.

Third, specify the parent table followed by a list of comma-separated columns to which the foreign key columns reference.

Finally, specify how foreign key maintains the referential integrity between the child and parent tables by using the ON DELETE and ON UPDATE clauses.  The reference_option determines action which MySQL will take when values in the parent key columns are deleted (ON DELETE) or updated (ON UPDATE).

MySQL has five reference options: CASCADE, SET NULL, NO ACTION, RESTRICT, and SET DEFAULT.

CASCADE: if a row from the parent table is deleted or updated, the values of the matching rows in the child table automatically deleted or updated.
SET NULL:  if a row from the parent table is deleted or updated, the values of the foreign key column (or columns) in the child table are set to NULL.
RESTRICT:  if a row from the parent table has a matching row in the child table, MySQL rejects deleting or updating rows in the parent table.
NO ACTION: is the same as RESTRICT.
SET DEFAULT: is recognized by the MySQL parser. However, this action is rejected by both InnoDB and NDB tables.
In fact, MySQL fully supports three actions: RESTRICT, CASCADE and SET NULL.

If you don’t specify the ON DELETE and ON UPDATE clause, the default action is RESTRICT.

MySQL FOREIGN KEY examples
Let’s create a new database called fkdemo for the demonstration.

CREATE DATABASE fkdemo;

USE fkdemo;
RESTRICT & NO ACTION actions
Inside the fkdemo database, create two tables categories and products:

CREATE TABLE categories(
    categoryId INT AUTO_INCREMENT PRIMARY KEY,
    categoryName VARCHAR(100) NOT NULL
) ENGINE=INNODB;

CREATE TABLE products(
    productId INT AUTO_INCREMENT PRIMARY KEY,
    productName varchar(100) not null,
    categoryId INT,
    CONSTRAINT fk_category
    FOREIGN KEY (categoryId) 
        REFERENCES categories(categoryId)
) ENGINE=INNODB;
The categoryId in the products table is the foreign key column that refers to the categoryId column in the  categories table.

Because we don’t specify any ON UPDATE and ON DELETE clauses, the default action is RESTRICT for both update and delete operation.