Miscellaneous linked server creation – aliases, other database providers and Excel

After publishing recent posts regarding linked servers I noticed there are still some topics about linked servers that are interesting but not so straightforward. So, I decided to describe them details to cover these points fully. This will include a lot of messing with connection strings – I recommend using http://www.connectionstrings.com as a reference.

Here we go:

  1. Create linked server with different name and Windows Authentication.

  2. This will require using OLE DB provider for SQL Server – we will use a different name (alias) for the linked server and provide a full connection string like:

    Provider=SQLNCLI10;Data Source=myServerAddress;Initial Catalog=myDataBase;Integrated Security=True;

    Don’t forget to choose correct security option – I vote for number two or three! 🙂

  3. Create linked server with automatic failover (for use with AlwaysOn/database mirroring).

  4. This is an extension of an idea above – we will use OLE DB Provider, a different name (alias) for the linked server and provide a full connection string with definitione of failover partner like:

    Provider=SQLNCLI10;Data Source=myServerAddress;Initial Catalog=myDataBase;Integrated Security=True;Failover Partner=mirrorServer

    Adding a Failover Partner parameter to the connection string allows the client to attempt automatic reconnection in case of automatic failover, but you have to keep the mirror server up-to-date with server objects, such as logins or Agent jobs, otherwise your application might behave erratically.

  5. Create linked server to MySQL Server.

  6. This one is more tricky. There is no official OLE DB driver for MySQL, so we have to use ODBC instead. It has to be downloaded from MySQL site, installed and then we need to define system ODBC before we can connect to MySQL. After that it’s just a matter of providing the right connection string, for example:

    Driver={MySQL ODBC 5.1 Driver};Server=myServerAddress;Database=myDataBase;User=myUsername;Password=myPassword;Option=3;

    Let’s try it. Installation of the driver is straightforward, so the fun starts with system ODBC creation. I have MySQL server ready and waiting at mysql5.beton.test with one database and a table inside that contains two records:

    mysql> use beton
    Database changed
    mysql> select * from table1;
    | idTable1 | table1col |
    |        1 | aaaaa     |
    |        2 | acbd      |
    2 rows in set (0.00 sec)

    First I have to create ODBC data source (through Administrative Tools -> ODBC Data Sources): MySQL-ODBC-Step2MySQL-ODBC-Step1

    – since I am on 64-bit system I am using 64-bit driver – you might check if you need 32-bit or 64-bit;
    – I set up System DSN so that SQL Server can use it;
    – I choose Unicode driver and then I provide connection details – data source name, description, target server and port, user and password to connect to MySQL server, and startup database;
    – upon test connection it says “Connection successful”.

    After that initial setup I can proceed with linked server creation in SQL Server. In SSMS, I go to New Linked Server and provide following parameters:

    – Linked Server Name: MYSQL5 (it’s an alias I want to use);
    – Provider – Microsoft OLE DB Provider for ODBC Drivers;
    – Data Source – target server name, mysql5.beton.test in my case;
    – test connection – it should succeed at this point.

    mysql5_linked_serverFew notes about it:

    1. I explicitly mentioned user/password in the connection string. This causes all connections to be done in the context of this user. I couldn’t manage to get the connection working when using user mapping or domain accounts (might be related to ODBC setup). I’ll work on that but I am also open to any ideas about how to get it done.
    2. This does not allow for browsing the catalog we are used to in case of remote SQL Server – you can see the tables, but you can’t simply do SELECT col1, col2 FROM table1. It is caused by incompatibility of metadata in information_schema, so you have to prepare your queries in MySQL and then use OPENQUERY, which has to be configured first via sp_configure:
    3. sp_configure 'ad hoc distributed queries', 1
      reconfigure with override

      Then you can query your MySQL server like

      select * from OPENQUERY([MYSQL5], 'select * from table1;')

      This is the warning light should start flashing. Since any user can use OPENQUERY and the permissions on the remote server are determined based on the ODBC/linked server definition, you really want to give the account you use to connect the least access rights possible, otherwise be prepared to face uncontrolled data modifications. Also, ad hoc distributed queries is a server setting – once you enable it, everyone can use it in all sorts of ways, so keep it in mind if you want to maintain control over your data (you can also try to use “Disallow ad hoc access” option of linked server provider, but remember that it affects all linked servers using this provider).

      In both cases of PostgreSQL and MySQL, you don’t see the structure on remote database in Object Explorer until you enable ‘Allow inprocess‘. If this option is disabled though, it should protect SQL Server from errors caused by the provider. However, I observed strange behavior of crash of SQL Server when trying to use ODBC linked server provider with “Allow inprocess” option disabled. Steps to reproduce:

      1. Install ODBC driver – I used official drivers (64-bit version) for MySQL and for PostgreSQL.
      2. Set up a System DSN and confirm it works successfully using test connection option.
      3. In SSMS, create a linked server for MySQL/PostgreSQL. Check that “Allow inprocess” option is unchecked in MSDASQL provider properties.
      4. Open new query window and run query similar to SELECT * FROM OPENQUERY(linked_server, ‘SELECT * FROM table’), where linked_server is the name of newly created linked server, and table is any table. this will give error like:
      5. Msg 7399, Level 16, State 1, Line 8
        The OLE DB provider "MSDASQL" for linked server "MYSQL5" reported an error. Access denied.
        Msg 7350, Level 16, State 2, Line 8
        Cannot get the column information from OLE DB provider "MSDASQL" for linked server "MYSQL5".
      6. Run this query two more times – first gives exactly the same error, but after second SQL Server crashes:
        Faulting application name: sqlservr.exe, version: 2011.110.3000.0, time stamp: 0x5082086a
        Faulting module name: ntdll.dll, version: 6.2.9200.16384, time stamp: 0x5010acd2
        Exception code: 0xc0000374
        Fault offset: 0x00000000000ea2b9
        Faulting process id: 0xdbc
        Faulting application start time: 0x01cec279fec9e95b
        Faulting application path: C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Binn\sqlservr.exe
        Faulting module path: C:\Windows\SYSTEM32\ntdll.dll
        Report Id: 665792a8-2e6d-11e3-93fd-00155d040513
        Faulting package full name: 
        Faulting package-relative application ID: 
      7. I experienced this on SQL Server 2012 SP1 builds 3000 and 3128, but I suspect it’s a bug in 2012 SP1 as I haven’t found this mentioned in changelists of all 2012 SP1 CUs. I also found similar topic on SQLServerCentral forums which also mentions build 3128. At first I suspected GDR update 3128 to be the cause, but after uninstalling it and going back to 2012 SP1 RTM the issue persists.
      8. The workaround is to enable “Allow inprocess” on MSDASQL linked server provider – after enabling it all those problems go away.

  7. Create linked server to PostgreSQL Server.

  8. Linking to PostgreSQL is similar to MySQL, first you have to install ODBC driver for PostgreSQL (available here), set up system ODBC and create linked server in reference to ODBC definition. Then query a linked server using OPENQUERY or four-part name.

  9. Create linked server to Excel/Access.

  10. There is a comprehensive guide to linking to Excel/Access available here. In short, you have to install data provider (Microsoft.Jet.OLEDB.4.0 for Excel up to 2003 or Microsoft.ACE.OLEDB.12.0 for 2007 or newer) and then use it when defining new linked server. Sample T-SQL code:

    EXEC sp_addlinkedserver
        @server = 'ExcelServer2',
        @srvproduct = 'Excel', 
        @provider = data_provider_name,
        @datasrc = filename,
        @provstr = 'excel_version;IMEX=1;HDR=YES;'

    In the pseudocode above, data_provider_name is the provider to be used (based on Excel/Access version), excel_version is Excel 8.0 for 2003 and Excel 12.0 for 2007 and above. For more details I recommend visiting mentioned link.

Summary – in this post I described how to use aliases for linked SQL Servers, how to set up a linked server for MySQL/PostgreSQL and Excel. I also showed an error I spotted for ODBC linked server provider in SQL Server 2012 SP1. As usual, I await comments/questions.

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