創建主鍵時，所相應的列假設沒有索引。數據庫默認會自己主動創建一個索引；假設對于列有索引，那么創建主鍵不會再創建索引。這里要注意，列值必須滿足主鍵的要求(唯一，非空)。簡單測試例如以下：

SQL> create table wxlun_pri(a number);

Table created.

SQL> alter table wxlun_pri add constraint PRIMARY_KEY_REPLY primary key (a);???? ------創建主鍵約束

Table altered.

SQL> select index_name from user_indexes where table_name='WXLUN_PRI';????? ------默認生成索引

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);??? ------創建一個一般索引，測試有反復值時加入主鍵約束

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);???? ------原索引有反復值。加入主鍵約束失敗
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

刪除反復值。加入主鍵約束

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>

總結

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