So now if you want to place AdventureWorks2008R2 sample database in a single-user mode, then write the code

ALTER DATABASE AdventureWorks2008R2 SET SINGLE_USER;

　　The above code would wait indefinitely if there is a lock on the database or users are connected to the database. So to overcome this situation, use the code below

ALTER DATABASE AdventureWorks2008R2 SET SINGLE_USER WITH ROLLBACK IMMEDIATE;

If the AUTO_UPDATE_STATISTICS_AYSYNC option for the database is ON, then you will be unable to place the database in single-user mode because the background thread that is used to update the statistics takes a connection against the database.

• The DBCC offers two repair modes:
  • REPAIR_REBUILD: This performs a repair that does not lead to any data loss.
  • REPAIR_ALLOW_DATA_LOSS: This performs a repair and fixes to the corrupted database structures, and also results in data loss.
• The following example will let you understand the whole concept, where I have used DBCC CHECKDB with REPAIR_ALLOW_DATA_LOSS option;

 ALTER DATABASE AdventureWorks2008R2 SET SINGLE_USER WITH ROLLBACK IMMEDIATE;

BEGIN TRANSACTION;

DBCC CHECKDB ('AdventureWorks2008R2', REPAIR_ALLOW_DATA_LOSS);

ALTER DATABASE AdventureWorks2008R2 SET MULTI_USER;

http://www.sqlservergeeks.com/repair-the-database-using-dbcc-checkdb/

https://msdn.microsoft.com/en-us/library/ms176064.aspx