关于创建主键和索引的关系一个小小測试

创建主键时，所相应的列假设没有索引，数据库默认会自己主动创建一个索引。假设对于列有索引，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)。简单測试例如以下：

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。測试有反复值时加入主键约束

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);     ------原索引有反复值，加入主键约束失败
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

删除反复值，加入主键约束

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>