1. BE PREPARED Error Handling & Optimizing T-SQL Code in SQL Server 2000
Stevan Popovski, MCSD
Interbank Clearing Systems,KIBS
2004 PASS SQL Server Academy
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

2. Error handling target Database consistency Quick error detection
Error cause identification
Scouting out error location
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

3. Developer’s prerequisites
Low level of confidence
High level of paranoia
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

4. Errors classification 1
Expected errors – trapped errors
(Raiserror, Return, Print, Select)
1. Information messages
2. Logical errors
Unexpected errors
1. Compile Error
(example: syntax error)
2. Run-time Error
(example: constraint violation)
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

5. Error message structure
CREATE PROCEDURE dbo.Demo1Proc1 AS
SELECT 'STRING'/2 GO
EXEC Demo1Proc1
Server: Msg 245, Level 16, State 1,
Procedure Demo1Proc1, Line 3
Syntax error converting the varchar value 'STRING' to a column of data type int.
Message number
Severity level
State
Procedure
Line
Message text
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

6. Error detection VB6 On Error Resume Next Go To ErrHandler VB.NET
Try , Catch, Finally,
End Try
SQL 2000
Return parameter
YUKON
Begin Try, End Try
Begin Catch, End Catch
0 - after successful execution
err.number – after statement that generate error
RETURN parameter
0 after successful execution of stored procedures
There is no way for error suppression in SQL Server 2000
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

7. Batch definition
A batch is a group of one or more Transact-SQL statements
sent at one time from an application to SQL Server for execution
Batch samples:
GO <group og T-SQL statements> GO
EXECUTE (“sql query”)
sp_executesql (“sql query”)
Execution of three nested procedures is one Batch
 Proc1  Proc2  Proc3
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

8. Scope definition
The current executing procedure, function or triger called by other procedure is scope
Proc1 – entire batch
Proc2 – part of batch – scope1
Proc3 – part of batch and part of scope1 – scope2
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

9. Error classification 2 Statement level error Scope level error
(depend on SQL Server “after error” behaviour)
Statement level error
Scope level error
Batch level error
Connection level error
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

10. Statement level error The current statement is aborted and rolled back
Execution continues with next statement
If transaction is open it is not rolled back
System function is equal to err.number
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

11. Statement level error Table is created
CREATE TABLE Demo2Table (p1 CHAR(8)) GO
INSERT INTO Demo2Table VALUES ('in db1')
INSERT INTO Demo2Table VALUES (‘more than 8')
INSERT INTO Demo2Table VALUES ('in db3')
SELECT * FROM Demo2Table
(1 row(s) affected)
Server: Msg 8152, Level 16, State 9, Line 1
String or binary data would be truncated.
The statement has been terminated.
(1 row(s) affected)
Table is created
First and third insert are executed
Select statement is executed p1
in db1
in db3
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

12. Statement level error - samples
Duplicate primary key
Inserting Null value in not null column
Check or Foreign Key violation
Calling non existed stored procedure
Permission denied
Declaring same cursor second time
Missing parameter in store procedure execution
Rollback or Commit without active transaction
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

13. Scope level error The current scope is aborted
Execution continues on the next statement in the calling scope
System function is set to error number
The aborted procedure does not have a return value
Variable that receive the return value is unaffected
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

14. Scope level error EXEC Demo3PrCaller CREATE PROCEDURE Demo3PrCaller AS
INT
= 7777
= Demo3ScopeAbort
PRINT 'Return' +
Procedure Demo3PrCaller, COMMIT or ROLLBACK TRANSACTION statement is missing.
Return
CREATE PROCEDURE Demo3ScopeAbort AS
CREATE TABLE #Temp (p1 int not null)
PRINT 'FIRST PRINT'
BEGIN TRANSACTION
INSERT INTO #Temp
SELECT * FROM TabNonExist
COMMIT TRANSACTION
PRINT 'SECOND PRINT'
FIRST PRINT
Procedure Demo3ScopeAbort, Invalid object name TabNonExist
SELECT ColNoEx FROM TabEx
Procedure Demo3ScopeAbort
COMMIT or ROLLBACK TRANSACTION statement is missing.
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy
Invalid column name 'ColNoEx'.

15. Batch level error The execution of the entire batch is aborted
Any open transaction is rolled back
is set,
you can catch a non-zero error value in the next batch
There is no way you can obstruct batch-abortion
in T-SQL code
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

16. Batch level error declare @Ret int exec @Ret = Demo4BatchAbort
is not availible in this batch
Return parameter is not set
int
= Demo4BatchAbort
select
First select
CREATE PROCEDURE Demo4BatchAbort AS
as INT
SELECT 'First select'
int
exec =
SELECT ' Second select'
<> 0
Server: Msg 8146, Level 16, State 1, Procedure Demo4WithoutError, Line 0
Procedure Demo4WithoutError has no parameters and arguments were supplied.
CREATE PROCEDURE Demo4WithoutError AS
SELECT 'There is no error in this procedure'
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

17. Batch level error - samples
Converting non-numeric string to numeric value
Superfluous parameter to stored procedure
Exceeding the maximum nesting-level (32)
If batch is deadlock victim
Running out of space for data file or transaction log
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

18. Connection level error
SQL Server terminates the connection
Open transactions are Rolled back
It is not possible to check
Connection-termination examples:
Errors in the SQL Server process
Protocol errors in the communication
Hardware problems
Network problems
Database corruptions
Errors in dll components called by sp_OAxxx procedures
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

19. Transactions A transaction is a sequence of operations performed
as a single logical unit of work
Atomicity
A transaction must be an atomic unit of work;
either all of its data modifications are performed,
or none of them is performed
Consistency
When completed, a transaction must leave all data in a consistent state.
Isolation
Modifications made by concurrent transactions must be isolated from the modifications made by any other
concurrent transactions
Durability
After a transaction has completed, its effects are permanently in place in the system
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

20. BEGIN - COMMIT – ROLLBACK TRANSACTION
A COMMIT always balances a BEGIN TRANSACTION by reducing the transaction count by one
A single ROLLBACK always rolls back the entire transaction
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

21. Atomicity ??? BEGIN TRANSACTION Correct INSERT
Icorrect UPDATE –Statement termination Error
Correct DELETE
COMMIT TRANSACTION
In default mode (XACT_ABORT OFF):
The first and third statement will be commited
Atomicity is conditional
Developers are responsible for transaction management
IF <> 0 ROLLBACK TRANSACTION.
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

22. Transaction mode Autocommit Transaction Mode Explicit Transaction Mode
Implicit Transaction Mode
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

23. Transaction Isolation Levels
Dirty Read
Non-Repeatable Read
Phantom
Read Uncommitted
Yes
Read Committed (default) No
Repeatable Read
Serializable
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

24. XACT_ABORT – enforce atomicity
SET XACT_ABORT ON
SET XACT_ABORT OFF
--Demo5Tab p1 char(4),p2 char(12)
BEGIN TRANSACTION
INSERT INTO Demo5Tab Values('A','B')
UPDATE Demo5Tab set P1 = 'very long string'
UPDATE Demo5Tab set P2 = rtrim(P2) + '- Updated'
COMMIT TRANSACTION GO
SELECT * FROM Demo5Tab
(1 row(s) affected)
<error description>
p1 p2
(0 row(s) affected)
(1 row(s) affected)
<error description>
p1 p2
A B- Updated
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

25. XACT_ABORT & Scope L.Error
SET XACT_ABORT ON
exec Demo6PrCaller go
select * from Demo6Tab
p1 p2
(2 row(s)affected)
CREATE PROCEDURE Demo6PrCaller AS
INT
BEGIN TRANSACTION
INSERT INTO Demo6Tab Values('1','1')
= 7777
= Demo6ScopeAbort
PRINT 'Return' +
COMMIT TRANSACTION
1 row(s) affected
Return
1 row(s) affected
CREATE PROCEDURE Demo6ScopeAbort AS
INSERT INTO Demo6Tab Values('2','2')
UPDATE TabNonExist set P1 = 'X'
UPDATE Demo6Tab set P2 = rtrim(P2) + '- Updated'
Invalid object name TabNonExist
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

26. Scope level error – DEMO XACT_ABORT is unable to force atomicity if error causes scope level error Conclusion XACT_ABORT can’t ensure database consistency, without checking system function XACT_ABORT can’t help to localize error
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

27. Creating and executing stored procedure
Query processor
Resolution
(deferred name resolution)
Create Procedure
Parsing
Insert Into syscomments
query optimizer
(execution plan )
Compile
error
execution plan
in memory
First Execution
Execution
Run time
Successful
execution
Non First Execution
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

28. DNR & CodePage 1251
One of the following statements will generate error
UPDATE TABLE TableName SET p1 = 1
UPDATE TABLE ТableName SET p1 = 1
Using similar letters but different ascii characters
increase the risk of errors caused by referencing
none existing objects.
In this example T <> Т
SELECT ASCII ('T')     English – Latin T 84  SELECT ASCII ('Т')     Macedonian – Cyrilic T 210
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

29. Error detection using system function @@Error (statement level error)
AS INT
EXEC ProcNonExist =
<> 0
SELECT 'ERROR ' +
ELSE SELECT 'THERE IS NO ERROR'
Server: Msg 2812, Level 16, State 62, Line 2
Could not find stored procedure 'ProcNonExist'.
ERROR
(1 row(s) affected)
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

30. Error detection using system function @@Error (batch level error)
CREATE TABLE Demo8Tab (p1 int) GO
INSERT INTO Demo8Tab
VALUES (‘this has to be int')
IF <> 0 SELECT 'ERROR'
ELSE SELECT 'OK'
Server: Msg 245, Level 16, State 1, Line 1
Syntax error converting the varchar value 'this has to be int' to a column of data type int.
-----
ERROR
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

31. Short life of @@Error system function
Declare name_Cursor cursor for select 'something'
Declare name_Cursor cursor for select 'something_else'
if <> 0 select
Server: Msg 16915, Level 16, State 1, Line 2
A cursor with the name 'name_Cursor' already exists.
INT
Declare name_Cursor cursor for select 'something'
Declare name_Cursor cursor for select 'something_else' =
<> 0
Server: Msg 16915, Level 16, State 1, Line 3
A cursor with the name 'name_Cursor' already exists.
16915
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

32. Pseudo Return parameter
ansi_nulls on
if null <> 0
select 'yes'
else
select 'no'
---- no
ansi_nulls off
if null <> 0
select 'yes'
else
select 'no'
----
yes
Pseudo Return parameter
INT
--it follow exec without error
= Demo10Proc1
= 0 Print 'OK'
Else Print ‘Error'
--sleduva scope abortion
= Demo10Proc2
Else Print 'Error' OK
<Error message …> OK
It seems like both procedures return zero,
But the second exec doesn’t affect return parameter
In this case it is not enough to check = 0
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

33. Solution for pseudo Return
INT
-- it follow exec without error
= Demo10Proc1 =
= 0 Print 'OK'
Else Print ‘Error'
-- it follow scope abortion
= Demo10Proc2 OK
<Error message…>
Error
COALESCE (expression1,...n)
Returns the first nonnull expression among its arguments
NULLIF ( expression , expression )
Returns a null value if the two specified expressions are equivalent
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

34. Return - status number -1 Missing object. -2 Data type error. -3
Process was chosen as deadlock victim. -4
Permission error. -5
Syntax error. -6
Miscellaneous user error. -7
Resource error, such as out of space. -8
Nonfatal internal problem. -9
System limit was reached.
-10
Fatal internal inconsistency.
-11
-12
Table or index is corrupt.
-13
Database is corrupt.
-14
Hardware error.
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

35. Handling “Error Handling”
SYSTEM STATISTICAL FUNCTIONS
DATABASE OPTIONS
ARITHABORT
ARITHIGNORE
ANSI_WARNINGS
NUMERIC_ROUNDABORT
ARITHABORT
ANSI_WARNINGS
Behavior ON
Abort statement only.
OFF
Abort batch.
Continue; value is NULL .
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

36. Hierarchical view of stored procedures
Clasification of stored procedures depend on nested level:
First Level (FL)
Procedure called by client application,
FL procedure calls ML and LL stored procedures
Middle Level (ML)
Nested procedure called by other stored procedure,
ML procedure calls ML or LL stored procedures
Last Level (LL)
Nested procedure called by FL or ML procedure,
LL procedure doesn’t call other stored procedures
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

37. Creating hierarchical view
Finding real dependencies between stored procedures:
Sysdepends inconsistency
Developing solution to find real dependencies
Creating hierarchy using dependencies:
Proc1 Proc2
Proc2 Proc3
Proc4 Proc5
Proc1
>---Proc2
>------Proc3
Proc4
>--Proc5
Hierarchical view benefits:
Error Handling
Control over data and business flow
Documentation
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

38. Last level procedure CREATE PROCEDURE Proc111 AS UPDATE Table1 …
SET @Err =
IF @Err <> 0
BEGIN [RAISERROR/XP_LOGEVENT]
RETURN
END
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

39. Middle level procedure
CREATE PROCEDURE Proc11 AS
UPDATE Table1 …
SET @Err =
IF @Err <> 0
BEGIN [RAISERROR/XP_LOGEVENT]
RETURN
END
= Proc111 =
<> 0
BEGIN [RAISERROR/XP_LOGEVENT]
RETURN
END
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

40. First level procedure CREATE PROCEDURE Proc1 AS BEGIN TRANSACTION …
UPDATE Table1 …
SET @Err =
IF @Err <> 0
BEGIN
ROLLBACK TRANSACTION
[RAISERROR/XP_LOGEVENT]
RETURN
END
= Proc11
= 0), 1001)
<> 0
BEGIN
ROLLBACK TRANSACTION
[RAISERROR/XP_LOGEVENT]
RETURN
END
COMMIT TRANSACTION
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

41. Several considerations
1. Getting native error versus keeping error location
2. What if Error occurs in error handling block ?
3. To log or not to log ?
4. To raise or not to raise ?
5. Controling transactions in first level procedures
6. Avoiding recompilations
7. How to trap errors in user defined functions ?
8. Is it the stress test enough to avoid error handling blocks ?
9. Is it possible to make bullet proof stored procedure ? …
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

42. Trapping errors with SQL Profiler
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

43. YUKON – Error Handling BEGIN TRY BEGIN Transaction SELECT ‘error’ / 0
COMMIT Transaction
END TRY
BEGIN CATCH TRAN_ABORT
int =
PRINT ' +
ROLLBACK
END CATCH
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

44. Links http://www.algonet.se/~sommar/error-handling-II.html
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy

45. THANK YOU !
Error Handling & Optimizing T-SQL Code in SQL Server * 205
June 25, 2004 – 9.45
2004 PASS SQL Server Academy