创建主键时，所对应的列如果没有索引，数据库默认会自动创建一个索引；如果对于列有索引，那么创建主键不会再创建索引。这里要注意，列值必须满足主键的要求(唯一，非空)，简单测试如下：
SQL> create table wxlun_pri(a number);
Table created.
SQL> alter table wxlun_pri add constraint PRIMARY_KEY_REPLY primary key (a); ------创建主键约束
Table altered.
SQL> select index_name from user_indexes where table_name='WXLUN_PRI'; ------默认生成索引
INDEX_NAME
------------------------------
PRIMARY_KEY_REPLY
SQL> alter table wxlun_pri drop constraint PRIMARY_KEY_REPLY;
Table altered.
SQL> select index_name from user_indexes where table_name='WXLUN_PRI';
no rows selected
SQL> create index idx_wxlun_pri on wxlun_pri(a); ------创建一个一般索引，测试有重复值时添加主键约束
Index created.
SQL> insert into wxlun_pri values(1);
1 row created.
SQL> /
1 row created.
SQL> commit;
Commit complete.
SQL> select * from wxlun_pri;
A
----------
1
1
SQL> alter table wxlun_pri add constraint PRIMARY_KEY_REPLY primary key (a); ------原索引有重复值，添加主键约束失败
alter table wxlun_pri add constraint PRIMARY_KEY_REPLY primary key (a)
*
ERROR at line 1:
ORA-02437: cannot validate (WXLUN.PRIMARY_KEY_REPLY) - primary key violated
删除重复值，添加主键约束
SQL> delete from wxlun_pri where rownum<2;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select * from wxlun_pri;
A
----------
1
SQL> alter table wxlun_pri add constraint PRIMARY_KEY_REPLY primary key (a); ------成功
Table altered.
SQL> select index_name from user_indexes where table_name='WXLUN_PRI';
INDEX_NAME
------------------------------
IDX_WXLUN_PRI
SQL> drop table wxlun_pri;
Table dropped.
SQL>