Module 12 Handling Errors in T-SQL Code

Module Overview Understanding T-SQL Error Handling Implementing T-SQL Error Handling Implementing Structured Exception Handling

Lesson 1: Understanding T-SQL Error Handling Where T-SQL Errors Occur Types of Errors What's in an Error? Error Severity Demonstration 1A: Error Types and Severity

Where T-SQL Errors Occur T-SQL Errors can occur in all phases  Syntax of a statement is checked  Object names are resolved  Statement is executed Errors can be handled in two ways  Error handling code in the database engine  Error handling code in the calling application

Types of Errors CategoryNotes Syntax ErrorsExecution is never started Object Resolution ErrorsExecution is never started Statement Terminating ErrorsExecution resumes at next statement Scope TerminatingExecution resumes at next statement after statement that called the scope Batch Terminating ErrorsAborts the batch Connection Terminating ErrorsDisconnects the session Server Terminating ErrorsStops SQL Server (Rare)

What's in an Error? Errors raised by the database engine have the following attributes: AttributeDescription Error numberEach error message has a unique error number Error MessageString containing diagnostic info about the cause of the error SeverityIndicates how serious the error is StateValue used to determine the location in code at which an error occurred Procedure NameName of the stored procedure or trigger in which the error occurred (if applicable) Line NumberIndicates in which line of a batch, stored procedure, trigger or function generated the error

Error Severity The severity of an error indicates the type of problem encountered by SQL Server Severity RangeDescription 0 to 9Informational messages 10Informational messages that return status information 11 to 16Errors that can be corrected by the user 17 to 19Software errors that cannot be corrected by the user 20 to 24Serious system errors 25SQL Server service terminating error

Demonstration 1A: Error Types and Severity In this demonstration you will: See how different types of errors are returned from T-SQL statements See the types of messages that are related to severe errors Query the sys.messages view and note which errors are logged automatically

Lesson 2: Implementing T-SQL Error Handling Raising Errors Using Errors and Transactions Transaction Nesting Errors Raising Custom Errors Creating Alerts When Errors Occur Demonstration 2A: T-SQL Error Handling

Raising Errors RAISERROR  Allows raising errors in code  Is used to return errors and messages back to applications using the same format as a system error RAISERROR can return either:  A user-defined error message created with sp_addmessage  A message string specified in the RAISERROR statement int = DB_ID(); sysname = DB_NAME(); RAISERROR (N'Current database ID:%d, database name: %s.', 10, -- Severity. 1, First substitution -- Second substitution argument. int = DB_ID(); sysname = DB_NAME(); RAISERROR (N'Current database ID:%d, database name: %s.', 10, -- Severity. 1, First substitution -- Second substitution argument.

Using Returns zero if the last statement executed correctly Returns the error number if the statement generated an error Changes value on the completion of each statement RAISERROR(N'Message', 16, 1); IF <> 0 PRINT 'Error=' + AS VARCHAR(8)); GO int; RAISERROR(N'Message', 16, 1); = <> 0 PRINT 'Error=' + AS VARCHAR(8)); RAISERROR(N'Message', 16, 1); IF <> 0 PRINT 'Error=' + AS VARCHAR(8)); GO int; RAISERROR(N'Message', 16, 1); = <> 0 PRINT 'Error=' + AS VARCHAR(8));

Errors and Transactions Most errors are statement terminating errors  Statement that caused them is rolled back  Execution continues with the next statement SET XACT_ABORT  When ON, errors that would abort the T-SQL statement will abort the batch and roll back any enclosing transaction as well  When OFF (default value), in most cases only the T-SQL statement that raised the error will be rolled back  Does not affect compile and syntax errors

Transaction Nesting Errors Any ROLLBACK causes all levels of transactions to be rolled back  Nested transactions are not supported in SQL Server  Autonomous transactions are not supported in SQL Server can be used to test the transaction level Stored procedures must have the same on entry and exit or error 286 is thrown  Common situation that occurs when attempting to nest transactions

Raising Custom Errors sp_addmessage allows adding custom error messages  Error number must be or above  Can be specific to a language EXECUTE sp_addmessage 61487, 10, 'Current DatabaseID: %d, Database Name: %s'; GO int = DB_ID(); sysname = DB_NAME(); RAISERROR EXECUTE sp_addmessage 61487, 10, 'Current DatabaseID: %d, Database Name: %s'; GO int = DB_ID(); sysname = DB_NAME(); RAISERROR

Creating Alerts When Errors Occur Alerts can be raised on errors that are logged Messages (including system messages) can be altered to be alert-raising by:  Using RAISERROR() WITH LOG  Altering a message to make it logged via sp_altermessage

Demonstration 2A: T-SQL Error Handling In this demonstration, you will see: How to raise errors How severity affects errors How to add a custom error message How to raise a custom error message That custom error messages are instance-wide How to use That system error messages cannot be raised

Lesson 3: Implementing Structured Exception Handling TRY CATCH Block Programming Error Handling Functions Catchable vs. Non-catchable Errors TRY CATCH and Transactions Errors in Managed Code Demonstration 3A: Deadlock Retry

TRY CATCH Block Programming Structured exception handling is more powerful than  Allows focus on the purpose of code rather than on error handling  TRY/CATCH blocks can be nested BEGIN CATCH must follow END TRY BEGIN TRY -- Generate divide-by-zero error. SELECT 1/0; END TRY BEGIN CATCH -- Execute the error retrieval routine. EXECUTE Error.GeneralHandler; END CATCH; BEGIN TRY -- Generate divide-by-zero error. SELECT 1/0; END TRY BEGIN CATCH -- Execute the error retrieval routine. EXECUTE Error.GeneralHandler; END CATCH;

Error Handling Functions CATCH blocks have richer options for capturing error information than was available through Error functions are able to return the error information throughout the CATCH block, not just for the first statement Error handlers can be enclosed in a stored procedure for reuse CREATE PROCEDURE Error.GeneralHandler AS SELECT ERROR_NUMBER() AS ErrorNumber, ERROR_SEVERITY() AS ErrorSeverity, ERROR_STATE() as ErrorState, ERROR_PROCEDURE() as ErrorProcedure, ERROR_LINE() as ErrorLine, ERROR_MESSAGE() as ErrorMessage; GO CREATE PROCEDURE Error.GeneralHandler AS SELECT ERROR_NUMBER() AS ErrorNumber, ERROR_SEVERITY() AS ErrorSeverity, ERROR_STATE() as ErrorState, ERROR_PROCEDURE() as ErrorProcedure, ERROR_LINE() as ErrorLine, ERROR_MESSAGE() as ErrorMessage; GO

Catchable vs. Non-catchable Errors Not all errors can be caught by TRY…CATCH blocks within the same scope  Compile errors  Statement level recompilation issues  Errors that are not able to be caught in their current scope can still be caught at the next outer scope

TRY CATCH and Transactions CATCH blocks do not automatically roll back transactions Code in the catch block needs to roll back any current transaction (but only if there is a transaction) SET XACT_ABORT ON; BEGIN TRY BEGIN TRAN; -- perform work COMMIT TRAN; END TRY BEGIN CATCH IF > 0 ROLLBACK TRAN; EXEC Error.GeneralHandler; RETURN -1; END CATCH; SET XACT_ABORT ON; BEGIN TRY BEGIN TRAN; -- perform work COMMIT TRAN; END TRY BEGIN CATCH IF > 0 ROLLBACK TRAN; EXEC Error.GeneralHandler; RETURN -1; END CATCH;

Errors in Managed Code Errors can be caught using standard.NET try/catch/finally handling within the high level languages used to create managed code All errors that are passed back from managed code to T- SQL will be wrapped in a 6522 error. Errors messages can contain nested error messages. SQL CLR messages need to be unpacked to find the inner exceptions rather than the 6522 error that they are wrapped in. Managed code could execute a RAISERROR T-SQL statement via a SqlCommand object.

Demonstration 3A: Deadlock Retry In this demonstration you will see how to use structured exception handling to retry deadlock errors

Lab 12: Handling Errors in T-SQL Code Exercise 1: Replace based error handling with structured exception handling Challenge Exercise 2: Add deadlock retry logic to the stored procedure (Only if time permits) Logon information Estimated time: 45 minutes

Lab Scenario A company developer asks you for assistance with some code he is modifying. The code was written some time back and uses simple T-SQL error handling. He has heard that structured exception handling is more powerful and wishes to use it instead.

Lab Review Why do we need to test for transaction state in a CATCH block? Why do we insert a delay within the retry logic for a deadlock?

Module Review and Takeaways Review Questions Best Practices