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  • 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>
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  • 原文地址:https://www.cnblogs.com/zfyouxi/p/5088513.html
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