1. Exception types In Oracle PL/SQL

2. Types of exceptions Named system exceptions –Raised as a result of an error in PL/SQL or RDBMS processing. Named programmer-defined exceptions –Raised as a result of errors expected in the application code. Unnamed system exceptions –Raised as a result of an error in PL/SQL or RDBMS processing, with codes, but no names. Unnamed programmer-defined exceptions. –These are raised in the server by the programmer.

3. Named system exceptions Oracle can handle: –CURSOR_ALREADY_OPENED (sqlcode = -6511) –DUP_VAL_ON_INDEX (sqlcode = -1) –INVALID_CURSOR (sqlcode = -1001) –INVALID_NUMBER (sqlcode = -1722) –LOGIN_DENIED (sqlcode = -1017) –NO_DATA_FOUND (sqlcode = +100) –TOO_MANY_ROWS (sqlcode = -1422) –…etc… These are named in the ‘standard’ package in pl/sql.

4. To handle these exceptions explicitly: These exception names do not need to be declared. To handle them explicitly, put a clause in the exception section: EXCEPTION When DUP_VAL_ON_INDEX dbms_output.put_line(‘record already there’); When OTHER dbms_output.put_line(‘error occurred’); END;

5. Named Programmer-Defined Exceptions Application-specific exceptions –E.g. Negative balance in account Team cannot play against itself Cannot stock a negative number of items Programmer can trap these errors and handle them. To do this: –Name the error –Check for the error and raise it –Handle the error in the EXCEPTION section

6. Example PROCEDURE calc_annual_sales (company_id_in IN company.company_id%TYPE) IS no_sales_for_company EXCEPTION; BEGIN -- Code here to check the number of sales -- a company has made. If none: raise no_sales_for_company; -- any other code EXCEPTION WHEN no_sales_for_company THEN dbms_output.put_line(company_id||’ has made no sales’); WHEN other THEN rollback work; END;

7. create or replace procedure add_corderline (onum in builder.corderline.corderno%type, scode in builder.corderline.stock_code%type, qtyreq in builder.corderline.quantityrequired%type) as invalid_quantity exception; begin if (qtyreq <= 0) then raise invalid_quantity; end if; insert into corderline values(qtyreq, onum, scode); exception when dup_val_on_index then dbms_output.put_line('primary key violation'); dbms_output.put_line(sqlcode||'--'|| sqlerrm); when invalid_quantity then dbms_output.put_line('quantity is invalid'); when others then dbms_output.put_line('unexpected error'); dbms_output.put_line(sqlcode||'--'|| sqlerrm); end;

8. Unnamed system exceptions These errors are not pre-named, but have a number. They will be raised automatically by the RDBMS. The EXCEPTION section handles them in the WHEN OTHER clause. To name an unnamed error in your application: –Give the error a name using a PRAGMA, or compiler directive, called EXCEPTION_INIT. –PL/SQL or RDBMS raise the error automatically. –Handle the error in a specially written WHEN clause in the exception section.

9. Unnamed system exceptions We’ve all seen errors that Oracle throws at us: –ERROR: ORA=12170: TNS: Connect timeout occurred –TNS Listener does not currently know of service requested in connect descriptor Note: –All of these errors have an error number: –e.g. ORA = 12170 means that the connection timeout occurred. These errors are RAISED automatically by the system, because they are system errors.

10. Example DECLARE deadlock_detected EXCEPTION; PRAGMA EXCEPTION_INIT(deadlock_detected, -60); BEGIN... -- Some operation that causes an ORA-00060 -- error (see locking) EXCEPTION WHEN deadlock_detected THEN -- handle the error END;

11. Unnamed Programmer-Defined Exceptions This is where the server (i.e. PL/SQL code) has declared and raised an application error and wants the client to be able to recognise it. When the server encounters an error, it returns the error code to the client. The client must declare the exception name and check for it after control has returned from the server.

12. Pragma EXCEPTION_INIT To handle error conditions that have no predefined name, you must use –the OTHERS handler or –the pragma EXCEPTION_INIT. A pragma –is a compiler directive that is processed at compile time, not at run time. –In PL/SQL, the pragma EXCEPTION_INIT tells the compiler to associate an exception name with an Oracle error number. –That lets you refer to any internal exception by name and to write a specific handler for it. –When you see an error stack, or sequence of error messages, the one on top is the one that you can trap and handle.

13. Pragma exception_init Coding –You code the pragma EXCEPTION_INIT in the declarative part of a PL/SQL block, subprogram, or package using the syntax PRAGMA EXCEPTION_INIT(exception_name, - Oracle_error_number); –where exception_name is the name of a previously declared exception –and the number is a negative value corresponding to an ORA- error number. –The pragma must appear somewhere after the exception declaration in the same declarative section.

14. Example DECLARE deadlock_detected EXCEPTION; PRAGMA EXCEPTION_INIT(deadlock_detected, -60); BEGIN... -- Some operation that causes an ORA-00060 error (see locking) EXCEPTION WHEN deadlock_detected THEN -- handle the error WHEN OTHER -- handle errors END;

15. Defining Your Own Error Messages: Procedure RAISE_APPLICATION_ERROR –lets you issue user-defined ORA- error messages from stored subprograms. –That way, you can report errors to your calling application and avoid returning unhandled exceptions.

16. Raise_application_error To call RAISE_APPLICATION_ERROR: raise_application_error(error_number, message[, {TRUE | FALSE}]); where error_number is a negative integer in the range -20000.. -20999 and message is a character string up to 2048 bytes long. If the optional third parameter is TRUE, – the error is placed on the stack of previous errors. If the parameter is FALSE (the default), –the error replaces all previous errors.

17. How to handle errors Declare to server Tell compiler (pragma exception init)RaiseHandle Named System Errorsxxx√ Named Application Errors√x√√ Unnamed System errors√√x√ Unnamed Application Errors√√√√