Primary key versus key

问题

When creating a mysql dump containing the structure of my database, one of the tables shows the following:

CREATE TABLE `completedTransactions` (
  `paymentId` int(10) unsigned NOT NULL,
  `timestamp` int(15) unsigned NOT NULL,
  `actionTaken` varchar(25) NOT NULL,
  `response` varchar(255) NOT NULL,
  `responseCode` int(5) NOT NULL,

  PRIMARY KEY  (`paymentId`,`timestamp`),
  KEY `paymentId` (`paymentId`),

The primary key is what I was expecting, but I'm unsure what the last line is about?

KEY `paymentId` (`paymentId`),

Is this related to an index?

回答1:

Yes, the KEY keyword is just an alias for the INDEX keyword.

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
  ...
  {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
      [index_option] ...

Source: MySQL Documentation: CREATE TABLE

回答2:

KEY is not unique, PRIMARY KEY and UNIQUE KEY are uniques.

回答3:

Quoting the documentation of CREATE TABLE :

KEY is normally a synonym for INDEX.
The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

来源：https://stackoverflow.com/questions/2199285/primary-key-versus-key