关于mysql中的key和index

1. 官方文档：http://dev.mysql.com/doc/refman/5.5/en/create-table.html

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.

A 

UNIQUE

 index creates a constraint such
that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a 

UNIQUE

 index
permits multiple

NULL

 values for columns
that can contain 

NULL

.

 A 

PRIMARY
KEY

 is a unique index where all key columns must be defined as 

NOT
NULL

. If they are not explicitly declared as 

NOT
NULL

, MySQL declares them so implicitly (and silently). A table can have only one 

PRIMARY
KEY

. The name of a 

PRIMARY KEY

 is
always 

PRIMARY

, which thus cannot be used
as the name for any other kind of index.

If you do not have a 

PRIMARY KEY

 and an
application asks for the 

PRIMARY KEY

 in
your tables, MySQL returns the first 

UNIQUE

 index
that has no 

NULL

 columns as the 

PRIMARY
KEY

.

In 

InnoDB

 tables, keep the 

PRIMARY
KEY

 short to minimize storage overhead for secondary indexes. Each secondary index entry contains a copy of the primary key columns for the corresponding row. (SeeSection 14.3.11,
“

InnoDB

 Table and Index Structures”.)

In the created table, a 

PRIMARY KEY

 is
placed first, followed by all 

UNIQUE

 indexes,
and then the nonunique indexes. This helps the MySQL optimizer to prioritize which index to use and also more quickly to detect duplicated

UNIQUE

 keys.

A 

PRIMARY KEY

 can be a multiple-column
index. However, you cannot create a multiple-column index using the

PRIMARY
KEY

 key attribute in a column specification. Doing so only marks that single column as primary. You must use a separate 

PRIMARY
KEY(index_col_name

,
...) clause.

 If a 

PRIMARY
KEY

 or 

UNIQUE

 index consists of
only one column that has an integer type, you can also refer to the column as 

_rowid

 in 

SELECT

 statements.

In MySQL, the name of a 

PRIMARY KEY

 is 

PRIMARY

.
For other indexes, if you do not assign a name, the index is assigned the same name as the first indexed column, with an optional suffix (

_2

, 

_3

, 

...

)
to make it unique. You can see index names for a table using 

SHOW
INDEX FROM tbl_name

.
See Section 13.7.5.23,
“

SHOW INDEX

Syntax”.

2. http://forums.macrumors.com/showthread.php?t=334697

Note that "primary" is called PRIMARY KEY not INDEX. 
KEY is something on the logical level, describes your table and database design (i.e. enforces referential integrity
...)
INDEX is something on the physical level, helps improve access time for table operations.
Behind every PK there is (usually) unique index created (automatically).

3. http://wenku.baidu.com/view/ff96febb1a37f111f1855bf8.html
在创建innoDB表时，在每张表中都只能有一个主键，如果在创建表时没有明确地定义主键（Primary Key），则innoDB引擎会自动按照如下方式选择或创建主键：首先看表中是否有非空唯一索引（Unique NOT NULL）如果有，则该列即为主键。 不符合上述条件，则自动创建一个6字节大小的RowID。