A deadlock in SQL Server occurs when two or more transactions are waiting for each other to release locks on resources they need to proceed. This creates a situation where none of the transactions can complete. SQL Server automatically detects deadlocks and resolves them by terminating one of the transactions, which is known as the deadlock victim.

Common Causes of Deadlocks

1. Resource Contention: Multiple transactions are trying to access the same resource at the same time.

2. Lock Escalation: SQL Server escalates fine-grained locks (like row locks) to coarser-grained locks (like table locks), potentially causing deadlocks.

3. Complex Transactions: Long-running and complex transactions that lock multiple resources.

4. Circular Waits: Each transaction is waiting for a resource locked by another transaction, forming a cycle.

Detecting Deadlocks

SQL Server provides tools to detect and analyze deadlocks:

• SQL Server Profiler: Capture deadlock graphs by creating a trace.

• Extended Events: Use extended events to capture detailed deadlock information.

• System Health Session: The default system health session captures deadlock graphs.

Example of Capturing Deadlocks Using Extended Events

Here's how you can set up an extended event to capture deadlocks:

CREATE EVENT SESSION [DeadlockEvent] ON SERVER
ADD EVENT sqlserver.deadlock_graph
ADD TARGET package0.event_file (SET filename = N'DeadlockEventFile')
WITH (MAX_MEMORY = 4096 KB, EVENT_RETENTION_MODE = ALLOW_SINGLE_EVENT_LOSS, MAX_DISPATCH_LATENCY = 30 SECONDS, MAX_EVENT_SIZE = 0 KB, MEMORY_PARTITION_MODE = NONE, TRACK_CAUSALITY = OFF, STARTUP_STATE = OFF);
GO

ALTER EVENT SESSION [DeadlockEvent] ON SERVER STATE = START;
GO

Resolving and Preventing Deadlocks

1. Indexing: Proper indexing can reduce the likelihood of deadlocks by minimizing the number of rows affected by transactions.

2. Transaction Design: Keep transactions short and simple. Avoid user interaction within transactions.

3. Consistent Access Order: Access resources in a consistent order across transactions to reduce the chances of circular waits.

4. Lock Hints: Use lock hints to control the locking behavior of SQL Server.

5. Isolation Levels: Use appropriate isolation levels. For example, snapshot isolation can help reduce deadlocks.

Example of Handling Deadlocks

You can use a retry logic to handle deadlocks in your application:

DECLARE @RetryCount INT = 0;
DECLARE @MaxRetries INT = 5;

WHILE @RetryCount < @MaxRetries
BEGIN
    BEGIN TRY
        BEGIN TRANSACTION;
        -- Your transaction code here
        COMMIT TRANSACTION;
        BREAK;
    END TRY
    BEGIN CATCH
        IF ERROR_NUMBER() = 1205 -- Deadlock error number
        BEGIN
            SET @RetryCount = @RetryCount + 1;
            WAITFOR DELAY '00:00:05'; -- Wait for 5 seconds before retrying
        END
        ELSE
        BEGIN
            ROLLBACK TRANSACTION;
            THROW;
        END
    END CATCH;
END

Understanding and managing deadlocks is crucial for maintaining the performance and stability of your SQL Server databases. If you have any specific scenarios or questions, feel free to ask!