以下内容整理自Oracle 官方文档
一 概念
A synonym is an alias for any table, view,materialized view, sequence, procedure, function, package, type, Java classschema object, user-defined object type, or another synonym. Because a synonymis simply an alias, it requires no storage other than its definition in thedata dictionary.
Oracle中同义词是任何表、视图、物化视图、序列、存储过程、函数、包、类型、JAVA类对象、用户定义类型,或是其他的同义词的别名。由于其只是一个别名，所以除了在
数据字典中的定义不占任何空间。
Synonyms are often used for security andconvenience. For example, they can do the following:
同义词常用于安全和方便。例如，它们可以做：
1. Mask the name and owner of anobject 伪装对象名称和其所有者。
2. Provide location transparency for remoteobjects of a distributed database 为分布式数据库远程对象提供位置透明性
3. Simplify SQL statements for databaseusers 简化数据库用户访问对象SQL语句
4. Enable restricted access similar tospecialized views when exercising fine-grained access control 当行使精细化访问控制时提供类似指定视图的访问限制
You can create both public and privatesynonyms. A public synonym is owned by the special user group named PUBLIC andevery user in a database can access it. A private synonym is in the schema of aspecific user who has control over its availability to others.
你可以创建public和private同义词。public同义词属于PUBLIC组，每个用户都可以访问。private同义词属于对象所有者，只有其显式授权后其他用户才可访问。
Synonyms are very useful in bothdistributed and nondistributed database environments because they hide theidentity of the underlying object, including its location in a distributedsystem. This is advantageous because if the underlying object must be renamedor moved, then only the synonym needs to be redefined. Applications based onthe synonym continue to function without modification.
同义词的优势体现在如果其底层的对象重命名或者转移，那么只需要重定义该同义词。基于该同义词的应用则无需任何修改。
Synonyms can also simplify SQL statementsfor users in a distributed database system. The following example shows how andwhy public synonyms are often created by a database administrator to hide theidentity of a base table and reduce the complexity of SQL statements. Assume thefollowing:
下面举例说明同义词是如何简化用户访问的：
A table called SALES_DATA is in the schemaowned by the user JWARD.
JWARD用户下有一张表 SALES_DATA
The SELECT privilege for the SALES_DATAtable is granted to PUBLIC.
PUBLIC组有SALES_DATA的查询权限
At this point, you have to query the tableSALES_DATA with a SQL statement similar to the following:
此时，你如果查询SALES_DATA表则需以下语句：
SELECT * FROM jward.sales_data;
Notice how you must include both the schemathat contains the table along with the table name to perform the query.
Assume that the database administratorcreates a public synonym with the following SQL statement:
假如数据库管理员创建了一个public 同义词：
CREATE PUBLIC SYNONYM sales FORjward.sales_data;
After the public synonym is created, youcan query the table SALES_DATA with a simple SQL statement:
你的语句将简化为：
SELECT * FROM sales;
Notice that the public synonym SALES hidesthe name of the table SALES_DATA and the name of the schema that contains thetable.
二、CREATE SYNONYM 创建同义词
1、语法结构：
2、前提条件：
To create a private synonym in your own schema, you must have the CREATE SYNONYM system privilege.
在自己模式下创建私有同义词需要CREATE SYNONYM权限。
To create a private synonym in another user's schema, you must have the CREATE ANY SYNONYM system privilege.
在其他用户模式下创建私有同义词需要CREATE ANY SYNONYM权限。
To create a PUBLIC synonym, you must have the CREATE PUBLIC SYNONYM system privilege.
创建公有同义词，需要有CREATE PUBLIC SYNONYM权限。
3、示例
sqlplus / as sysdba
CREATE SYNONYM offices
FOR hr.locations;
GRANT SELECT ON hr.locations to SCOTT;
CREATE PUBLIC DATABASE LINK l_hr
CONNECT TO hr IDENTIFIED BY hr
USING 'orcl';
CREATE PUBLIC SYNONYM emp_table
FORHR.employees@l_hr;
GRANT SELECT ON HR.employees to SCOTT;
conn scott/tiger@orcl
SELECT count(*) from sys.offices;
select count(*) from emp_table;
三、DROP SYNONYM 删除同义词
1、语法结构：
2、前提条件：
To drop a private synonym, either the synonym must be in your own schema or you must have the DROP ANY SYNONYM system privilege.
删除私有同义词需要有DROP ANY SYNONYM权限。
To drop a PUBLIC synonym, you must have the DROP PUBLIC SYNONYM system privilege.
删除公有同义词需要有DROP PUBLIC SYNONYM权限。
3、示例
3.1删除public同义词，必须加public关键字：
SYS@orcl>DROP SYNONYM emp_table;
DROPSYNONYM emp_table
*
第 1 行出现错误:
ORA-01434:要删除的专用同义词不存在
SYS@orcl>DROP PUBLIC SYNONYM emp_table;
同义词已删除。
3.2删除private同义词：
DROP SYNONYM offices;
四、Q&A 问答
Q: 可以对同义词做INSERT/UPDATE/DELETE操作吗？
A:
SCOTT@orcl> UPDATE sys.offices t SETt.city='Shanghai' WHERE location_id=1000;
UPDATE sys.offices t SET t.city='Shanghai'WHERE location_id=1000
*
第 1 行出现错误:
ORA-01031: 权限不足
SYS@orcl> grant update on hr.locationsto scott;
授权成功。
SCOTT@orcl> /
已更新 1 行。
SO: 用户对同义词的操作权限都是基于对其底层对象有哪些操作权限。
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Dylan Presents.
