MySQL Federated primary Key

Single Field primary key

MySQL single field primary key I believe everyone is familiar with it. Is the primary key for the primary data row of a field, such as the following table

Table 'user' (int (all-in-one),key (user_id));  

USER_ID as a single-field primary key, stating that no user's user_id is the same, a user_id that identifies a user.

Multi-field Federated primary key

Look at this table again

 create  table  ' user   "(username  varchar  (24  ), password  char  (32   Span style= "COLOR: #0000ff" >varchar  (40  ",  primary  key  " Span style= "COLOR: #000000" > (Username,email));  

Here Username,email is used as the Federated primary Key, in order to identify a user, only the username and email are present together, and can not be identified at the same time.

Example

usernamepasswordemail       zhangsan111111    [email protected]zhangsan111111    [email protected]    lisi       111111    [email protected]

These three records are legal records, and the core of a multi-field federated primary key is (all fields can uniquely determine a record, like the username,email above).

MySQL Federated primary Key