SQL连接表-它是否只需要主键还是只需要唯一键？

Question

I have created an application for the CakePHP framework which uses a join table.

I am unsure as to whether it is neccessary that I need a primary key to uniquley identify each row for the join table, as shown in the first block of SQL.

Do the two fields need to be set as unique keys or can they both be set as primary keys and I remove id as the primary key?

I was also asked why declaring atomic primary keys using a table constraint rather than a column constraint, does this mean I shouldn't set unique keys for a join table?

CREATE TABLE IF NOT EXISTS `categories_invoices` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `category_id` int(11) NOT NULL,
  `invoice_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `category_id` (`category_id`,`invoice_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=163 ;

I was thinking the solution is possibly to set both keys as unique and remove the primary key as shown here:

 CREATE TABLE IF NOT EXISTS `categories_invoices` (
      `category_id` int(11) NOT NULL,
      `invoice_id` int(11) NOT NULL,
      UNIQUE KEY `category_id` (`category_id`,`invoice_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=latin1;

I did in fact test deleting the primary key 'id' for the join table leaving only 'category_id' and 'invoice_id' and the application still worked. This has left both fields as unique fields within the join table. Is this in fact the correct practice?

Answer 1

You don't need both. The compound unique key can replace the Primary key (unless the Cake framework cannot deal with compound Priamry Keys):

 CREATE TABLE IF NOT EXISTS categories_invoices (
      category_id int(11) NOT NULL,
      invoice_id int(11) NOT NULL,
      PRIMARY KEY (category_id, invoice_id)
    ) 
    ENGINE = MyISAM 
    DEFAULT CHARSET = latin1 ;

---

It's also good to have another index, with the reverse order, besides the index created for the Primary Key:

   INDEX (invoice_id, category_id)

---

If you want to define Foreign Key constraints, you should use the InnoDB engine. With MyISAM you can't have Foreign Keys. So, it would be:

 CREATE TABLE IF NOT EXISTS categories_invoices (
      category_id int(11) NOT NULL,
      invoice_id int(11) NOT NULL,
      PRIMARY KEY (category_id, invoice_id),
      INDEX invoice_category_index (invoice_id, category_id)
    ) 
    ENGINE = InnoDB  
    DEFAULT CHARSET=latin1 ;

---

If Cake cannot cope with composite Primary Keys:

 CREATE TABLE IF NOT EXISTS categories_invoices (
      id int(11) NOT NULL AUTO_INCREMENT,
      category_id int(11) NOT NULL,
      invoice_id int(11) NOT NULL,
      PRIMARY KEY (id),
      UNIQUE KEY category_invoice_unique (category_id, invoice_id),
      INDEX invoice_category_index (invoice_id, category_id)
    ) 
    ENGINE = InnoDB  
    DEFAULT CHARSET=latin1 ;

Answer 2

There is nothing wrong with the second method. It is referred to as a composite key and is very common in database design, especially in your circumstance.

http://en.wikipedia.org/wiki/Relational_database#Primary_keys