Database Modelling. Lecture 8 (b): PL/SQL Exceptions Handling 1/6/2015 1

Transcription

1 Database Modelling Lecture 8 (b): PL/SQL Exceptions Handling 1/6/2015 1

2 This lecture will cover: Overview - The need for exception handling in PL/SQL - How exceptions are handled in Oracle PL/SQL - How to write user-defined exceptions - How to write a catch-all exception handler - How program flow of control differs when exceptions are encountered 1/6/2015 2

3 Transactions in Procedures On the surface very easy. If everything goes well (at end): COMMIT; If things go badly (at end): ROLLBACK; Must do COMMIT or ROLLBACK to release resources Problem is controlling bad outcomes: Handling exceptions Giving useful feedback 1/6/2015 3

4 Sample PL/SQL Procedure Let us consider a procedure to add vaccination data To generate: one successful run three unsuccessful runs Here we closely look at the results of these runs 1/6/2015 4

5 ADD_VACC procedure 1/2 SQL> CREATE OR REPLACE PROCEDURE add_vacc (pat_id in char, vis_vdate in date, 2 vis_act in number, vac_vacc in char) 3 AS 4 BEGIN 5 DBMS_OUTPUT.PUT_LINE ('Insert attempted'); 6 insert into vaccinations(pid,vdate,action,vaccinated) 7 values(pat_id,vis_vdate,vis_act,vac_vacc); 8 DBMS_OUTPUT.PUT_LINE ('Insert succeeded'); 1/6/2015 5

6 ADD_VACC procedure 2/2 9 EXCEPTION 10 WHEN others THEN DBMS_OUTPUT.PUT_LINE ('error'); 11 DBMS_OUTPUT.PUT_LINE ('Insert rejected'); 12 END; 13 / Procedure created. 1/6/2015 6

7 Successful Run 1a SQL> execute add_vacc('2','16-dec-1999',3,'cholera'); Insert attempted Insert succeeded PL/SQL procedure successfully completed. SQL> select * from vaccinations 2 where pid = '2' and action = 3; PID VDATE ACTION VACCINATED AUG-96 3 polio 2 16-DEC-99 3 cholera SQL> commit; Commit complete. 1/6/2015 7

8 Successful Run 1b No error messages Message PL/SQL procedure successfully completed again. Any exception raised during run has been properly handled Does not necessarily mean data has been added successfully COMMIT should have been last line in the procedure Here user has made decision to commit 1/6/2015 8

9 Unsuccessful Run 1a SQL> execute add_vacc('2','16-dec-1999', 3,'cholera'); Insert attempted error Insert rejected PL/SQL procedure successfully completed. /* error on primary key constraint */ 1/6/2015 9

10 Unsuccessful Run 1b Error message is vague Message PL/SQL procedure successfully completed again ROLLBACK should have been last line in the procedure 1/6/

11 Unsuccessful Run 2a SQL> execute add_vacc('2','17-dec-1999', 1,'cholera'); Insert attempted error Insert rejected PL/SQL procedure successfully completed. /* error on foreign key constraint */ 1/6/

12 Unsuccessful Run 3a SQL> execute add_vacc('2','16-dec-1999','4',cholera); BEGIN add_vacc('2','16-dec-1999','4',cholera); END; * ERROR at line 1: ORA-06550: line 1, column 38: PLS-00201: identifier 'CHOLERA' must be declared ORA-06550: line 1, column 7: PL/SQL: Statement ignored 1/6/

13 Unsuccessful Run 3b Error message returned: ORA indicates non-declared identifier Parameter value CHOLERA is not in quotes Therefore taken as variable Not declared to system No message about successful completion As exception not handled Unsatisfactory outcome Great care needed with parameter values 1/6/

14 Exception Handling in PL/SQL Essential part of any program Particularly needed for updates open-ended nature of user inputs But also needed for searches e.g. may not find any matching data An exception is raised when an operation: fails to perform normally A non-handled exception leads to program failure, stale locks and system degradation 1/6/

15 Exceptions Raised With input particularly Cannot specify all Oracle error codes in advance Too many codes to specify Some rule exceptions though can be emphasised Need specific exceptions And general (catch-all) exceptions 1/6/

16 Complete PL/SQL procedure template CREATE OR REPLACE PROCEDURE proc_name (parameters) AS local_vars BEGIN executable_code EXCEPTION exception_code END; / 1/6/

17 Example Procedure - part 1 CREATE OR REPLACE PROCEDURE add_patient (pat_id in char, pat_name in char, pat_address in char, pat_dobirth in date, pat_regdate in date) AS pid_too_high exception; PRAGMA EXCEPTION_INIT(pid_too_high,-20000); BEGIN DBMS_OUTPUT.PUT_LINE ('Insert attempted'); insert into patient(pid,pname,address,dobirth,date_reg) values(pat_id,pat_name,pat_address,pat_dobirth,pat_regdate); IF pat_id > '500' THEN RAISE pid_too_high; END IF; DBMS_OUTPUT.PUT_LINE ('Insert successful'); COMMIT; 1/6/

18 Example Procedure - part 2 EXCEPTION WHEN pid_too_high THEN DBMS_OUTPUT.PUT_LINE ('pid too high'); ROLLBACK; WHEN others THEN DBMS_OUTPUT.PUT_LINE ( insert rejected ); ROLLBACK; END; / 1/6/

19 Explanation (1) pid_too_high exception; variable pid_too_high of type exception (value true or false) PRAGMA EXCEPTION_INIT(pid_too_high,-20000); instruction to compiler associates error code with this type of error Oracle reserves error codes in the range of to for user-defined errors IF pat_id > '500' THEN RAISE pid_too_high; END IF; enforces a business rule that pid <= 500 by raising exception pid_too_high when this state 1/6/2015 occurs 19

20 Explanation (2) EXCEPTION opens exception handling part of procedure WHEN THEN ; defines actions when a particular exception occurs 1/6/

21 Flow of Control (1) If no exception raised insert is performed commit takes place procedure terminates with successful message If specific exception for business rule raised insert is performed exception pid_too_high is raised in IF code execution of main code immediately finishes code in EXCEPTION section after WHEN pid_too_high is executed including rollback procedure terminates with successful message 1/6/

22 Flow of Control (2) If another exception raised (on insert e.g. primary key violation) insert is not performed exception is raised in procedure execution of main code immediately finishes exception handler others is executed procedure terminates with successful message Always need the catch-all exception handler others 1/6/

23 Catch-all Exceptions Make last EXCEPTION handler: WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE ('Operation failed ' 'SQLCODE: ' SQLCODE); DBMS_OUTPUT.PUT_LINE ('SQL Error Message: ' SQLERRM); ROLLBACK; Note: is concatenation in output the SQLCODE for failure is displayed together with an explanatory message in SQLERRM 1/6/

24 Summary Following this lecture, you should know: - Why exception handling is needed - How exceptions are handled in Oracle PL/SQL - How to write user-defined exceptions, e.g. WHEN pid_too_high THEN - How to write a general (catch-all) exception handler WHEN OTHERS THEN - How program flow of control differs when exceptions are encountered 1/6/