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Synonyms (Database Engine)

  • Artikkeli

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

A synonym is a database object that serves the following purposes:

  • Provides an alternative name for another database object, referred to as the base object, that can exist on a local or remote server.

  • Provides a layer of abstraction that protects a client application from changes made to the name or location of the base object.

For example, consider the Employee table of Adventure Works, located on a server named Server1. To reference this table from another server, Server2, a client application would have to use the four-part name Server1.AdventureWorks.Person.Employee. Also, if the location of the table were to change, for example, to another server, the client application would have to be modified to reflect that change.

To address both these issues, you can create a synonym, EmpTable in a dedicated or existing schema, RemoteObjects, on Server2 for the Employee table on Server1. Now, the client application only has to use the two-part name, RemoteObjects.EmpTable, to reference the Employee table Server1. Also, if the location of the Employee table changes, you will have to modify the synonym, EmpTable, to point to the new location of the Employee table. Because there is no ALTER SYNONYM statement, you first have to drop the synonym, RemoteObjects.EmpTable, and then re-create the synonym with the same name, but now point the synonym to the new location of the Employee table.

A synonym belongs to a schema, and like other objects in a schema, the name of a synonym must be unique. You can create synonyms for the following database objects:

Assembly (CLR) stored procedure

Assembly (CLR) scalar function

Replication-filter-procedure

SQL scalar function

SQL inline-tabled-valued function

View

Assembly (CLR) table-valued function

Assembly (CLR) aggregate functions

SQL table-valued function

SQL stored procedure

Table* (User-defined)

*Includes local and global temporary tables

Note

Four-part names for function base objects are not supported.

A synonym cannot be the base object for another synonym, and a synonym cannot reference a user-defined aggregate function.

The binding between a synonym and its base object is by name only. All existence, type, and permissions checking on the base object is deferred until run time. Therefore, the base object can be modified, dropped, or dropped and replaced by another object that has the same name as the original base object. For example, consider a synonym, dbo.MyContacts, that references the Person.Contact table in Adventure Works. If the Contact table is dropped and replaced by a view named Person.Contact, MyContacts now references the Person.Contact view.

References to synonyms are not schema-bound. Therefore, a synonym can be dropped at any time. However, by dropping a synonym, you run the risk of leaving dangling references to the synonym that was dropped. These references will only be found at run time.

Synonyms and Schemas

If you have a default schema that you do not own and want to create a synonym, you must qualify the synonym name with the name of a schema that you do own. For example, if you own a schema S1, but S2 is your default schema and you use the CREATE SYNONYM statement, you must prefix the name of the synonym with the schema S1, instead of naming the synonym by using a single-part name. For more information about how to create synonyms, see CREATE SYNONYM (Transact-SQL).

Granting Permissions on a Synonym

Only synonym owners, members of db_owner, or members of db_ddladmin can grant permission on a synonym.

You can GRANT, DENY, and REVOKE all or any of the following permissions on a synonym:

CONTROL

EXECUTE

SELECT

UPDATE

DELETE

INSERT

TAKE OWNERSHIP

VIEW DEFINITION

Using Synonyms

You can use synonyms in place of their referenced base object in several SQL statements and expression contexts. The following columns contain a list of these statements and expression contexts:

SELECT

UPDATE

EXECUTE

INSERT

DELETE

Sub-selects

When you are working with synonyms in the contexts previously stated, the base object is affected. For example, if a synonym references a base object that is a table and you insert a row into the synonym, you are actually inserting a row into the referenced table.

Note

You cannot reference a synonym that is located on a linked server.

You can use a synonym as the parameter for the OBJECT_ID function; however, the function returns the object ID of the synonym, not the base object.

You cannot reference a synonym in a DDL statement. For example, the following statements, which reference a synonym named dbo.MyProduct, generate errors:

ALTER TABLE dbo.MyProduct  
   ADD NewFlag int null;  
EXEC ('ALTER TABLE dbo.MyProduct  
   ADD NewFlag int null');

The following permission statements are associated only with the synonym and not the base object:

GRANT

REVOKE

DENY

Synonyms are not schema-bound and, therefore, cannot be referenced by the following schema-bound expression contexts:

CHECK constraints

Default expressions

Schema-bound views

Computed columns

Rule expressions

Schema-bound functions

For more information about schema-bound functions, see Create User-defined Functions (Database Engine).

Getting Information About Synonyms

The sys.synonyms catalog view contains an entry for each synonym in a given database. This catalog view exposes synonym metadata such as the name of the synonym and the name of the base object. For more information, see sys.synonyms (Transact-SQL).

By using extended properties, you can add descriptive or instructional text, input masks, and formatting rules as properties of a synonym. Because the property is stored in the database, all applications that read the property can evaluate the object in the same way. For more information, see sp_addextendedproperty (Transact-SQL).

To find the base type of the base object of a synonym, use the OBJECTPROPERTYEX function. For more information, see OBJECTPROPERTYEX (Transact-SQL).

Examples

The following example returns the base type of a synonym's base object that is a local object.

USE tempdb;  
GO
CREATE SCHEMA SynSchema
GO  
CREATE SYNONYM SynSchema.MyEmployee   
FOR AdventureWorks2022.HumanResources.Employee;  
GO  
SELECT OBJECTPROPERTYEX(OBJECT_ID('SynSchema.MyEmployee'), 'BaseType') AS BaseType;

The following example returns the base type of a synonym's base object that is a remote object located on a server named Server1.

EXECUTE sp_addlinkedserver Server1;  
GO  
CREATE SYNONYM SynSchema.MyRemoteEmployee  
FOR Server1.AdventureWorks2022.HumanResources.Employee;  
GO  
SELECT OBJECTPROPERTYEX(OBJECT_ID('MyRemoteEmployee'), 'BaseType') AS BaseType;  
GO

Related Content

Create Synonyms
CREATE SYNONYM (Transact-SQL)
DROP SYNONYM (Transact-SQL)