First of all this is not normal to start SQL Server in single user mode. So if you are reading this blog by getting this as a search result, you might be in a tough situation. In some situations, like restoring system database or during disaster recovery you may need to start SQL in single user mode.

Okay, so you have started SQL Server in single user mode by specifying start-up parameter “m” either by net start MSSQLServer /m or via command prompt sqlservr.exe –m –sInstanceName but when you are trying to connect via any tool (SQLCMD, OSQL, SQL Server Management Studio or any other) you are welcomed by error message.

TITLE: Microsoft SQL Server Management Studio
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Error connecting to ‘(local)\SQL2k8R2’.
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ADDITIONAL INFORMATION:
Login failed for user ‘Contoso\demouser’. Reason: Server is in single user mode. Only one administrator can connect at this time. (Microsoft SQL Server, Error: 18461)
For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&EvtSrc=MSSQLServer&EvtID=18461&LinkId=20476
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BUTTONS
OK
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Error message is clear that someone “else” has made connection to SQL Server and due to /m switch, SQL is not letting you get in. Now, logically there are two ways to fix this problem.

• Find out who is connecting before you and stop that application (difficult in real/disaster time)
• Ask SQL Server to not to allow anyone except me.

Second one sounds more easy. So let’s discuss that.

If you want to know the root cause of “why its in single user mode” then go back and look for SQL Error Log and you may find something as below.

In above Errorlog, we can see additional start-up parameter and warning that SQL is in single user mode.

Essentially we want to start in single use mode and no one else except you should be able to connect. Books online has explained this clearly that you can append m parameter with the client application name. Here are few example

• Only SQLCMD should be able to connect then it would be m”SQLCMD”
• If you want to use Management studio only then it would be m"Microsoft SQL Server Management Studio – Query".

Let’s see it in action. I can add start-up parameter in multiple ways. My favorite is staring SQL from command line as a service, My instance name is SQL2K8R2 so below would be the command

Net Start MSSQL$SQL2K8R2 /m”SQLCMD”

If you have default instance than it would be

Net Start MSSQLServer /m”SQLCMD”

Once you have started SQL Service in Single use mode then only SQLCMD application can connect and other connection would get error message.

Let’s see what we nave in ERRORLOG

2013-12-06 09:13:50.08 Server      Registry startup parameters:
     -d E:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\master.mdf
     -e E:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Log\ERRORLOG
     -l E:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\DATA\mastlog.ldf
2013-12-06 09:13:50.08 Server      Command Line Startup Parameters:
     -s "MSSQLSERVER"
     -m "SQLCMD"

…

2013-12-06 09:13:50.80 spid4s      Warning ******************
2013-12-06 09:13:50.80 spid4s      SQL Server started in single-user mode. This an informational message only. No user action is required.

….

2013-12-06 09:14:32.93 Logon       Error: 18461, Severity: 14, State: 1.
2013-12-06 09:14:32.93 Logon       Login failed for user ‘Contoso\demouser’. Reason: Server is in single user mode. Only one administrator can connect at this time. [CLIENT: <local machine>]

It’s important to note that string after –m parameter is case-sensitive. This means that if you give sqlcmd (all lower case) then connection can’t be made. If we want to use management studio then the parameter would be –m”Microsoft SQL Server Management Studio – Query”. This is the exactly same name as in program_name when you look at sys.processes or sys.dm_exec_sessions

Hopefully this blog would help you in making a connection to SQL Server without stopping application, changing password, disabling account as there were the tricks I have seen to get into SQL when only one connection can be made and unfortunately that’s not you.

Cheers,
Balmukund Lakhani
Twitter @blakhani
Author: SQL Server 2012 AlwaysOn – Paperback, Kindle