删除外键
alter table t1 drop primary key;
alter table t1 add primary key(id,name);
改变表字段
alter table t1 change id id int(3);
alter table t1 modify id int(3);
alter table t1 drop primary key;
alter table t1 add primary key(id,name);
alter table t1 change id id int(3);
alter table t1 modify id int(3);