这个SQL语句该怎么改？

[mwhero]

create table ddl
(ddsid char(6) not null,
name char(3) not null
);
commit;
alter table ddl
constraint PK_ddsid primary key(ddsid);
commit;

[vivianfdlpw]

alter table ddl
add constraint PK_ddsid primary key(ddsid);

[gw_delphi]

create table AAA
(
a1 NUMBER(6) not null,
b1 NUMBER(4) not null,

constraint AAA_PK primary key (a1)
)
==================================
alter table AAA
drop constraint AAA_PK cascade
;
alter table AAA
add constraint AAA_PK primary key (a1)

[Eric_1999]

create table ddl
(ddsid char(6) not null,
name char(3) not null
);
commit;
alter table ddl
add constraint PK_ddsid primary key(ddsid);
commit;