oracle: OCA-047-题解与实验(2）

最新推荐文章于 2021-04-06 14:25:18 发布

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最新推荐文章于 2021-04-06 14:25:18 发布

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分类专栏： oracle与c++开发文章标签： oracle dictionary user sql table object

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本文分析了Oracle数据库中USER_SYNONYMS与USER_OBJECTS表的特性，解释了私有同义词的概念及其作用。同时，指出了关于系统用户、动态性能视图及数据字典查询的常见误解，强调了对象所有权的重要性。

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题目：

逐条分析吧：

A：正确，USER_SYNONYMS describes the private synonyms (synonyms owned by the current user). Its columns (except for OWNER) are the same as those in ALL_SYNONYMS.通过他的描述即可判断。在数据库里同义词就相当于表的别名的功能，用一个别名来代表一个表。如：

SQL> create or replace synonym syn1 for jobs;
 
Synonym created
 
SQL> select * from user_synonyms;
 
SYNONYM_NAME  TABLE_OWNER  TABLE_NAME  DB_LINK
------------- ------------ ----------- -------------
SYN1          HR           JOBS                           
 
SQL> desc user_synonyms;
Name         Type          Nullable Default Comments                                      
------------ ------------- -------- ------- --------------------------------------------- 
SYNONYM_NAME VARCHAR2(30)                   Name of the synonym                           
TABLE_OWNER  VARCHAR2(30)  Y                Owner of the object referenced by the synonym 
TABLE_NAME   VARCHAR2(30)                   Name of the object referenced by the synonym  
DB_LINK      VARCHAR2(128) Y                Database link referenced in a remote synonym  
 
SQL> select * from syn1;
 
JOB_ID     JOB_TITLE                           MIN_SALARY MAX_SALARY
---------- ----------------------------------- ---------- ----------
AD_PRES    President                                20080      40000
AD_VP      Administration Vice President            15000      30000
AD_ASST    Administration Assistant                  3000       6000
FI_MGR     Finance Manager                           8200      16000

创建synonym的语句格式是：

CREATE [ OR REPLACE ] [ PUBLIC ] SYNONYM
   [ schema. ] synonym
   FOR [ schema. ] object [ @ dblink ] ;

B：错误，应该是sys用户，而不是system用户；

SQL> select count(*) from dba_tables where owner='SYSTEM';
 
  COUNT(*)
----------
       156
 
SQL> select count(*) from dba_tables where owner='SYS';
 
  COUNT(*)
----------
       954
 
SQL>

C：错误，动态性能视图并不是全部都开放给所有用户；
D：错误，不是created by the user only 而是owned by the user only，原文档描述是：USER_OBJECTS describes all objects owned by the current user. Its columns are the same as those in ALL_OBJECTS；
E：正确，DICTIONARY contains descriptions of data dictionary tables and views. 用户查询时只会列出用户可以访问的数据字典行；