I have table name persons,

```
CREATE TABLE Persons (
    P_Id int NOT NULL PRIMARY KEY,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255)
)

```

Now i am planning to add the primary key on column 2 Lastname, so i tried alter command: 

```
ALTER TABLE Persons
ADD CONSTRAINT pk_LastName PRIMARY KEY (LastName)

```

When i run the alter command i have error message: 1068 Multiple primary key defined.

Could you please help me how to fix this ?

Thanks,
Palani