how to create two primary key with alter table

I have table broker wi开发者_运维技巧th this form

broker
{
broker_code char(10) primary key

.

.

.
}

and i have add to it another primary key with this form

code int nut null auto_increment

and wrote this query

ALTER TABLE broker ADD code INT  NOT NULL AUTO_INCREMENT,
  ADD PRIMARY KEY (code);

but it this error.

multiply primary key defined.

what can i do?

You can't have two primary keys... However, you can have a "candidate" key or Unique that also prevents duplication on other criteria... Such as a person in a human resources application. You can have an internal "employee ID" which is the primary, but also a candidate key on a person's Social Security Number which SHOULD never be duplicated... and if so, it would through up a red-flag to H/R to verify who a person is.

This code below has a composite primary key with alter table.

ALTER TABLE ex_table
ADD PRIMARY KEY (`name`,`id`)

I'm not sure because I can't see the structure of your current table 'broker'. But my guess is that you already defined another Primary Key.

You are only allowed to have 1 primary key per table.