Learn SQL Synonyms: Simplify Database Object References

Synonyms in SQL Server are powerful database objects that act as alternative names for other database objects. These base objects can reside either on your local server or a remote one. Think of a synonym as a nickname or alias that simplifies how you refer to database elements, offering a layer of abstraction and flexibility to your database management.

Understanding Synonyms in SQL Server

At its core, a synonym provides an alternate name for another database object, known as the base object. This base object could be a table, view, stored procedure, function, or other database element. Synonyms are schema-bound objects, meaning they exist within a specific database schema, just like tables or views. The name you choose for a synonym must be unique within that schema.

Consider a scenario where you frequently access the Employee table in the AdventureWorks database, located on a server named Server1. Without a synonym, you’d need to use the cumbersome four-part name Server1.AdventureWorks.Person.Employee every time you wanted to query this table from another server, say Server2. This not only makes your queries lengthy and harder to read but also tightly couples your applications to the physical location of the Employee table.

Synonyms come to the rescue by allowing you to create a more user-friendly and stable reference. You could create a synonym named EmpTable within a schema like RemoteObjects on Server2, pointing to the Employee table on Server1. Now, your client applications on Server2 can simply use the two-part name RemoteObjects.EmpTable to access the Employee table.

Why Use Synonyms? Key Benefits Explained

Synonyms offer two primary advantages that contribute significantly to better database design and application maintainability:

Abstraction: Synonyms shield your applications from the underlying structure and location of database objects. If the base object, like our Employee table, moves to a different server or its name changes, you only need to update the synonym definition. Your applications, still referencing the synonym, remain unaffected. This decoupling is crucial for reducing application maintenance and deployment headaches when database changes occur.
Simplicity: Using synonyms simplifies complex object names, especially when dealing with remote servers or deeply nested schemas. Instead of remembering and typing lengthy four-part names, developers can work with shorter, more intuitive synonym names. This improves code readability and reduces the chances of errors.

Creating Synonyms: A Practical Guide

Creating a synonym in SQL Server is straightforward using the CREATE SYNONYM statement. Here’s the basic syntax:

CREATE SYNONYM schema_name.synonym_name
FOR server_name.database_name.schema_name.base_object_name;

Let’s break down the components:

schema_name.synonym_name: Specifies the schema and name you want to assign to your synonym.
server_name.database_name.schema_name.base_object_name: This is the four-part name of the base object you are referencing. If the base object is on the local server, you can omit the server_name.

Example: To create the EmpTable synonym as discussed earlier, you would execute the following SQL statement on Server2:

CREATE SYNONYM RemoteObjects.EmpTable
FOR Server1.AdventureWorks.Person.Employee;

Important Considerations:

Permissions: Only synonym owners, members of the db_owner database role, or members of the db_ddladmin database role can grant permissions on a synonym.
Object Types: You can create synonyms for various database objects, including tables (user-defined, local, and global temporary), views, stored procedures (SQL and CLR), functions (scalar, inline table-valued, and CLR), and aggregate functions (CLR). However, you cannot create a synonym for another synonym or a user-defined aggregate function.

Managing and Using Synonyms Effectively

Once created, synonyms behave much like the base objects they represent in many SQL statements.

Permissions: You can manage permissions on synonyms using GRANT, DENY, and REVOKE statements, controlling access to operations like SELECT, INSERT, UPDATE, DELETE, EXECUTE, and more. These permissions are managed independently of the base object’s permissions.

Usage: Synonyms can be used in SELECT, INSERT, UPDATE, DELETE, and EXECUTE statements, as well as in subqueries. When you perform operations on a synonym, you are actually interacting with the underlying base object. For instance, inserting data into a synonym that points to a table will insert rows into the table itself.

Limitations:

DDL Statements: You cannot use synonyms in Data Definition Language (DDL) statements like ALTER TABLE. For example, you cannot alter a synonym to point to a different base object; you must drop and recreate it.
Linked Servers: Synonyms located on linked servers are not supported.
Schema Binding: Synonyms are not schema-bound. This means that dropping a synonym will not invalidate dependent objects, but references to the dropped synonym will fail at runtime. Schema-bound contexts like check constraints, default expressions, schema-bound views, computed columns, rule expressions, and schema-bound functions cannot directly reference synonyms.

Retrieving Information About Synonyms

SQL Server provides system views and functions to help you gather information about synonyms:

sys.synonyms Catalog View: This system view contains metadata about each synonym in the database, including the synonym name and the base object name. You can query this view to list synonyms and their corresponding base objects.
OBJECTPROPERTYEX Function: You can use the OBJECTPROPERTYEX function to determine the base type of the base object referenced by a synonym. This is useful for understanding what kind of object a synonym is pointing to.

Examples:

To find the base type of a synonym named MyEmployee in the SynSchema schema, you can use:

SELECT OBJECTPROPERTYEX(OBJECT_ID('SynSchema.MyEmployee'), 'BaseType') AS BaseType;

For a synonym MyRemoteEmployee referencing a remote object, the process is similar:

SELECT OBJECTPROPERTYEX(OBJECT_ID('MyRemoteEmployee'), 'BaseType') AS BaseType;

Conclusion: Embracing Synonyms for Database Efficiency

Synonyms are a valuable tool in SQL Server for simplifying database object references, enhancing abstraction, and improving the maintainability of database applications. By using synonyms, you can create a more flexible and user-friendly database environment, shielding applications from the complexities of object locations and names. Learning to leverage synonyms effectively can significantly streamline your SQL Server development and administration tasks.