Deadlock and Trace Flags 1204 and 1222
When you are looking for deadlocks there are, like most things in SQL Server, more than one way to find the deadlocks. This article is specifically focused on using the SQL Server ERRORLOG file located in the LOG directory for the SQL instance. If you used the defaults on SQL Server 2012, this log would be at this location:
C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Log\ERRORLOG
For SQL Server 2014 it would be here:
C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQLSERVER\MSSQL\Log\ERRORLOG
and in similar locations for other versions of SQL Server.
What is a Deadlock?
A deadlock is a specific case in SQL Server and other databases where, in the simple case of two transactions, the first transaction gets stuck waiting on blocking from the second transaction, and the second transaction gets stuck waiting on blocking for the first transaction.
Imagine traffic so jammed up that one lane of travel is blocking another lane of travel, and vice versa. The deadlock in SQL Server is similar to this, but instead of traffic being blocked, it is queries and transactions being blocked.
Eventually in traffic when cars are blocking, one will give up and back out, and the other will be allowed to drive through. The one that backs out is known as the deadlock victim.
When one query is chosen as the deadlock victim, it looks something like this: