1.普通的主键，直接alter table x drop primary key;

        2.有auto_increment:  alter table x modify PrimaryKeyName int    : To cancel auto_increment

                                                 alter table x drop primary key;                       : Drop the primary key

        3.同时存在外键约束： alter table x drop foreign key;                         : Drop foreign key constraint

                                                alter table x drop primary key;                        : Drop primay key!

      OK，行了！