Partitioning Problem

[sireesh beemineni]

My table structure is give below

CREATE TABLE IF NOT EXISTS `employee` (
`emp_id` int(11) NOT NULL AUTO_INCREMENT,
`emp_name` varchar(20) NOT NULL,
`emp_sal` int(8) NOT NULL,
`emp_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
UNIQUE KEY `emp_id` (`emp_id`)
) ENGINE=MyISAM ;

i want to partition the table by emp_date wise.
So that i write the below query to do that task.

ALTER TABLE employee PARTITION BY RANGE (UNIX_TIMESTAMP(emp_date)) (
PARTITION P0 VALUES LESS THAN (1136073600),
PARTITION P1 VALUES LESS THAN (1167609600),
PARTITION P2 VALUES LESS THAN (1199145600),
PARTITION P3 VALUES LESS THAN (1230768000),
PARTITION P4 VALUES LESS THAN (1262304000 ),
PARTITION P8 VALUES LESS THAN (MAXVALUE)
);

If I run this query i am gettin the Error.
#1503 - A PRIMARY KEY must include all columns in the table's partitioning function


Please help me in this aspect.