With some applications there may be a need to pull data from other instances of SQL Server or from other databases within the same instance. Often this can be achieved by having multiple connections from your application pointing to each of these data sources. This is great for this one application, but what if there is a need to do this within SQL Server or for stored procedures or views that are called from several applications?
SQL Server offers the functionality to reference objects within the database you are working or to reference objects in another database or even a different instance of SQL Server. This is referred to as four-part naming. The reason for this name is that there can be four parts that are used to reference the object as the following shows:
For SQL Server 2000
The four parts are defined as follows:
For SQL Server 2005 and later
The four parts are defined as follows. The only difference is that owner is now referred to as schema.
In some cases, your SQL Server code may only reference the object name such as:
SELECT * FROM sysobjects
Owner or Schema
To further qualify this we can also specify the owner or schema of the object:
SELECT * FROM dbo.sysobjects
To take this a step further we can also reference the database that the object resides in such as:
SELECT * FROM master.dbo.sysobjects
And lastly, we can reference this table on a totally different server by specifying the server name. You would need to have a linked server setup to reference on an object on a totally different instance of SQL Server.
SELECT * FROM test1.master.dbo.sysobjects
The ability to reference an object this way opens up several possibilities whether it be within the same database or on other databases or other servers. One example is the use of the owner or schema part. This alone allows you to have the same object name in your database several times and with the owner or schema reference you can now signify which object you actually want to use in your code.
Also, if you have a standard database that may be used by multiple databases, by using the database part you can now have multiple databases pull in this standard data by using the database part of the four-part name.
And lastly, you can even separate your data even further by using the fourth part of the name that references the server. One thing to note is that you will need to setup linked servers in order to use this option. More information about linked servers can be found here for SQL Server 2000 and for SQL Server 2005 and later. Another thing to note is that there may be some performance issues if you are linking servers across your LAN or WAN and also if you are pulling in a lot of data.
- Take a look at your code to see if there are areas that could benefit from this four-part naming structure
- Another option to look at to retrieve data from another server or data source is OPENROWSET
- Look at the use of SYNONYMS in SQL Server 2005 or later to simplify four-part naming
- Also, take a look at ownership chaining and security/permission issues
Last Update: 11/6/2006
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