MS SQL servers always provided remote access to servers through RPC mechanisms, but they had the limitation of being confined to invoking procedures on remote locations. A linked server (a virtual server) may be considered a more flexible way of achieving the same thing, with the added benefits of remote table access and distributed queries. Microsoft manages the link mechanism via OLE DB technology. Specifically, an OLE DB datasource points to the specific database that can be accessed using OLEDB.
In this article we will be creating a MySQL linked server on SQL Server 2008 and querying a database [TestMove] table shown in the next listing . In reviewing the previous article it may be noticed that the Employees tables was moved to MySQL database TestMove. In running the commands from the mysql> prompt it is assumed that the MySQL Server has been started.
Listing 1: employees table in TestMove
mysql> show tables;
| Tables_in_testmove |
| employees |
1 row in set (0.09 sec)
Creating an ODBC DSN for MySQL
In the previous article on MySQL Servers cited earlier, a DSN was created for moving data. Essentially the same DSN can be used. Herein follows a brief review of the DSN MySQL_Link created along the same lines as in the previous referenced article. The ODBC driver used for creating this ODBC DSN is the one installed on the machine when the MySQL Server was installed as shown.
The final interactive window where you may test the connectivity is shown in the next figure. You may notice that the database Testmove has been named in the ODBC DSN. The name MySQL_LINK is the ODBC DSN.
When you close the window after clicking the OK button, a ODBC DSN item will be added to the System DSN tab of the ODBC wizard as shown.
Steps to create a linked server from Management Studio
Right click the Linked Servers node to display a drop-down menu as shown in the next figure.
Click on New Linked Server...item. This brings up the New Linked Server window as shown. The window is all empty except for a default Provider.
The very first thing to do is to provide a name for this linked server. Herein it is LINKED_ MYSQL. We will be using one of the providers [Microsoft OLE DB Provider for ODBC] supported by SQL Server 2008. You may access the list of OLE DB Providers in the Providers sub-folder of the Linked Servers. Provide the other details as shown in the next figure. Make sure they are entered exactly as shown or according to how you have created the database on MySQL Server.
Click on the Security list item under General in the left. This opens the 'Security' page of the New Linked Server wizard as shown. Change the login from the default "Be made without using a security context" to "Be made using this security context". Enter remote login. In this case it is "root" for the remote login and the password is the one used during the ODBC DSN (also the password for server authentication) creation.
Make no changes to the Server Options page. Click OK. This creates a linked server Linked_MySQL as shown expanded in the Linked Server's node as shown. You may need to right click and refresh the Linked Servers' node to see the new linked server. As you can see in the figure, the 'User' tables are not displayed.
Running Queries and reviewing results
Running system stored procedures can provide various levels of information and the database can be queried using the four part syntax and the openquery() method.
Information on the linked server
It is easy to find how the linked server is configured using system stored procedure sp_linkedsrvlogin on the SQL Server 2008. Open a Query window from File | New | Query Current Connection to open the query window and type in the following statement. The next figure shows the statement as well as the returned values. SQL Server 2008 querying has the intellisense report and this must be put to good use.
This shows all servers both local and remote as shown in the next figure.
Information about the tables on the remote server can also be accessed by running a stored procedure. Executing the stored procedure sp_tables_ex as shown in the next figure (which displays the statement and the result of executing the stored procedure) can be used to obtain table information.
Querying the table on the database
Data in the table on the linked server can be queried using the openquery() function. The syntax for this function shown next is very simple.
openquery ('linked server', 'query')
The next figure shows the result of running the openquery() function on the Linked_MySQL linked server.
Although it should be possible to query the linked server using the four part syntax as in:
Select * from LINKED_MYSQL...employees
The above statement returns an error. This is probably a limitation of a combination of MSDASQL and the ODBC driver which does not provide the schema information correctly(this is just the author's opinion).
Are Remote Procedure Calls (RPC) allowed?
The easiest way to test this is to send out a call by running the following query against the linked server.
Execute('Select FirstName, LastName from employees') at Linked_MYSQL
If the linked server is not configured for RPC, then the result you get by running the above query is as shown in the next figure.
Turn on RPC
Earlier on we skipped the Server Options page of the linked server. Back in the Management Studio right click linked server LINKED_MYSQL and from the drop-down choose to look at the properties at the bottom of the list. This brings up the LINKED_MYSQL properties window as shown. Click on Server Options. In the Server Options page change the values of RPC and RPCOUT to true, default for both being false.
Now run the query that produced the error previously. The result is displayed in the next figure.
You might have noted that only two columns were returned from the employees table. This was deliberate as trying to get all the column would produce an error due partly to the data types of data stored in the table and their compatibility with MSDASQL and the ODBC driver (Again, an author's opinion).
Creating Linked Server using TSQL
While the linked server can be created using the built-in wizard of the Management Studio, it can also be created using TSQL statements as in the following listing (run both statements, the first one creates the linked server and the second the logins).
The article described the steps involved in configuring a MySql Linked server on SQL Server 2008 using the built-in New Linked Server wizard as well as TSQL. Method to query the linked server as well as enabling RPC were described.
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