SQL finding Gaps and Islands

http://www.manning.com/nielsen/SampleChapter5.pdf
or
http://www.manning.com/nielsen/nielsenMEAP_freeCh5.pdf
or
http://www.sql.co.il/books/insidetsql2005/Inside%20Microsoft%20SQL%20Server%202005%20T-SQL%20Querying%20(0-7356-2313-9)%20-%20Chapter%2006%20-%20Aggregating%20and%20Pivoting%20Data.pdf

Rethrow SQL error

Not very interesting, but I use it all the time.

BEGIN TRY

 begin tran

    -- RAISERROR with severity 11-19 will cause execution to 
    -- jump to the CATCH block.
    RAISERROR ('Error raised in TRY block.', -- Message text.
               16, -- Severity.
               1 -- State.
               );
               
 commit tran
END TRY
BEGIN CATCH
    DECLARE @ErrorMessage NVARCHAR(4000);
    DECLARE @ErrorSeverity INT;
    DECLARE @ErrorState INT;

    SELECT 
        @ErrorMessage = ERROR_MESSAGE(),
        @ErrorSeverity = ERROR_SEVERITY(),
        @ErrorState = ERROR_STATE();

 rollback tran

    -- Use RAISERROR inside the CATCH block to return error
    -- information about the original error that caused
    -- execution to jump to the CATCH block.
    RAISERROR (@ErrorMessage, -- Message text.
               @ErrorSeverity, -- Severity.
               @ErrorState -- State.
               );
END CATCH;