Question

I have a table called Order_List that contains a list of orders. Each column contains a foreign key that references the primary key of an order in the order table. My question is, what is the best way to declare these foreign keys. Using MySQL Workbench I found two ways…

Method #1

CREATE  TABLE IF NOT EXISTS 'mydb'.'Order_List' (
    'idOrder_List' INT UNSIGNED NOT NULL AUTO_INCREMENT ,
    'orderID01' INT UNSIGNED NULL ,
    'orderID02' INT UNSIGNED NULL ,
    'orderID03' INT UNSIGNED NULL ,
    'orderID04' INT UNSIGNED NULL ,
    PRIMARY KEY ('idOrder_List') ,
    INDEX 'fk_Order_List_1' ('orderID01' ASC, 'orderID02' ASC, 'orderID03' ASC, 'orderID04' ASC) ,
    CONSTRAINT 'fk_Order_List_1'
      FOREIGN KEY ('orderID01' , 'orderID02' , 'orderID03' , 'orderID04' )
      REFERENCES 'mydb'.'Order' ('idOrder' , 'idOrder' , 'idOrder' , 'idOrder' )
      ON DELETE NO ACTION
      ON UPDATE NO ACTION)
    ENGINE = InnoDB

Method #2

  CREATE  TABLE IF NOT EXISTS 'mydb'.'Order_List' (
    'idOrder_List' INT UNSIGNED NOT NULL AUTO_INCREMENT ,
    'orderID01' INT UNSIGNED NULL ,
    'orderID02' INT UNSIGNED NULL ,
    'orderID03' INT UNSIGNED NULL ,
    'orderID04' INT UNSIGNED NULL ,
    PRIMARY KEY ('idOrder_List') ,
    INDEX 'fk_Order_List_1' ('orderID01' ASC) ,
    INDEX 'fk_Order_List_2' ('orderID02' ASC) ,
    INDEX 'fk_Order_List_3' ('orderID03' ASC) ,
    INDEX 'fk_Order_List_4' ('orderID04' ASC) ,
    CONSTRAINT 'fk_Order_List_1'
      FOREIGN KEY ('orderID01' )
      REFERENCES 'mydb'.'Order' ('idOrder' )
      ON DELETE CASCADE
      ON UPDATE CASCADE,
    CONSTRAINT 'fk_Order_List_2'
      FOREIGN KEY ('orderID02' )
      REFERENCES 'mydb'.'Order' ('idOrder' )
      ON DELETE NO ACTION
      ON UPDATE NO ACTION,
    CONSTRAINT 'fk_Order_List_3'
      FOREIGN KEY ('orderID03' )
      REFERENCES 'mydb'.'Order' ('idOrder' )
      ON DELETE NO ACTION
      ON UPDATE NO ACTION,
    CONSTRAINT 'fk_Order_List_4'
      FOREIGN KEY ('orderID04' )
      REFERENCES 'mydb'.'Order' ('idOrder' )
      ON DELETE NO ACTION
      ON UPDATE NO ACTION)
    ENGINE = InnoDB

The first method combines the declaration. The second method splits it up. Which one is better? Are there any inherent weaknesses to one over the other? Thanks!

Answer 1

This isn’t the same FK.

• One ensures that the combination of the 4 columns in any OrderList row exist in a single row row in 'mydb'.'Order'
• The other ensures that the 4 column values in any OrderList row exist independently of the others in some 'mydb'.'Order' row