| Oracle® Database PL/SQL Language Reference 11g Release 1 (11.1) Part Number B28370-01 |
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| Oracle® Database PL/SQL Language Reference 11g Release 1 (11.1) Part Number B28370-01 |
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View PDF |
Run-time errors arise from design faults, coding mistakes, hardware failures, and many other sources. Although you cannot anticipate all possible errors, you can plan to handle certain kinds of errors meaningful to your PL/SQL program.
With many programming languages, unless you disable error checking, a run-time error such as stack overflow or division by zero stops normal processing and returns control to the operating system. With PL/SQL, a mechanism called exception handling lets you bulletproof your program so that it can continue operating in the presence of errors.
Topics:
In PL/SQL, an error condition is called an exception. Exceptions can be internally defined (by the run-time system) or user defined. Examples of internally defined exceptions include division by zero and out of memory. Some common internal exceptions have predefined names, such as ZERO_DIVIDE and STORAGE_ERROR. The other internal exceptions can be given names.
You can define exceptions of your own in the declarative part of any PL/SQL block, subprogram, or package. For example, you might define an exception named insufficient_funds to flag overdrawn bank accounts. Unlike internal exceptions, user-defined exceptions must be given names.
When an error occurs, an exception is raised. That is, normal execution stops and control transfers to the exception-handling part of your PL/SQL block or subprogram. Internal exceptions are raised implicitly (automatically) by the run-time system. User-defined exceptions must be raised explicitly by RAISE statements, which can also raise predefined exceptions.
To handle raised exceptions, you write separate routines called exception handlers. After an exception handler runs, the current block stops executing and the enclosing block resumes with the next statement. If there is no enclosing block, control returns to the host environment. For information on managing errors when using BULK COLLECT, see "Handling FORALL Exceptions (%BULK_EXCEPTIONS Attribute)".
Example 11-1 calculates a price-to-earnings ratio for a company. If the company has zero earnings, the division operation raises the predefined exception ZERO_DIVIDE, the execution of the block is interrupted, and control is transferred to the exception handlers. The optional OTHERS handler catches all exceptions that the block does not name specifically.
Example 11-1 Run-Time Error Handling
DECLARE
stock_price NUMBER := 9.73;
net_earnings NUMBER := 0;
pe_ratio NUMBER;
BEGIN
-- Calculation might cause division-by-zero error.
pe_ratio := stock_price / net_earnings;
DBMS_OUTPUT.PUT_LINE('Price/earnings ratio = ' || pe_ratio);
EXCEPTION -- exception handlers begin
-- Only one of the WHEN blocks is executed.
WHEN ZERO_DIVIDE THEN -- handles 'division by zero' error
DBMS_OUTPUT.PUT_LINE('Company must have had zero earnings.');
pe_ratio := NULL;
WHEN OTHERS THEN -- handles all other errors
DBMS_OUTPUT.PUT_LINE('Some other kind of error occurred.');
pe_ratio := NULL;
END; -- exception handlers and block end here
/
The last example illustrates exception handling. With better error checking, you can avoided the exception entirely, by substituting a null for the answer if the denominator was zero, as shown in the following example.
DECLARE
stock_price NUMBER := 9.73;
net_earnings NUMBER := 0;
pe_ratio NUMBER;
BEGIN
pe_ratio :=
CASE net_earnings
WHEN 0 THEN NULL
ELSE stock_price / net_earnings
end;
END;
/
Because reliability is crucial for database programs, use both error checking and exception handling to ensure your program can handle all possibilities:
Add exception handlers whenever errors can occur.
Errors are especially likely during arithmetic calculations, string manipulation, and database operations. Errors can also occur at other times, for example if a hardware failure with disk storage or memory causes a problem that has nothing to do with your code; but your code still needs to take corrective action.
Add error-checking code whenever bad input data can cause an error.
Expect that at some time, your code will be passed incorrect or null parameters, that your queries will return no rows or more rows than you expect.
Test your code with different combinations of bad data to see what potential errors arise.
Make your programs robust enough to work even if the database is not in the state you expect.
For example, perhaps a table you query will have columns added or deleted, or their types changed. You can avoid such problems by declaring individual variables with %TYPE qualifiers, and declaring records to hold query results with %ROWTYPE qualifiers.
Handle named exceptions whenever possible, instead of using WHEN OTHERS in exception handlers.
Learn the names and causes of the predefined exceptions. If your database operations might cause particular ORA- errors, associate names with these errors so you can write handlers for them. (You will learn how to do that later in this chapter.)
Write out debugging information in your exception handlers.
You might store such information in a separate table. If so, do it by invoking a subprogram declared with the PRAGMA AUTONOMOUS_TRANSACTION, so that you can commit your debugging information, even if you roll back the work that the main subprogram was doing.
Carefully consider whether each exception handler should commit the transaction, roll it back, or let it continue.
No matter how severe the error is, you want to leave the database in a consistent state and avoid storing any bad data.
Using exceptions for error handling has several advantages. With exceptions, you can reliably handle potential errors from many statements with a single exception handler:
Example 11-2 Managing Multiple Errors with a Single Exception Handler
DECLARE
emp_column VARCHAR2(30) := 'last_name';
table_name VARCHAR2(30) := 'emp';
temp_var VARCHAR2(30);
BEGIN
temp_var := emp_column;
SELECT COLUMN_NAME INTO temp_var FROM USER_TAB_COLS
WHERE TABLE_NAME = 'EMPLOYEES'
AND COLUMN_NAME = UPPER(emp_column);
-- processing here
temp_var := table_name;
SELECT OBJECT_NAME INTO temp_var FROM USER_OBJECTS
WHERE OBJECT_NAME = UPPER(table_name)
AND OBJECT_TYPE = 'TABLE';
-- processing here
EXCEPTION
-- Catches all 'no data found' errors
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE
('No Data found for SELECT on ' || temp_var);
END;
/
Instead of checking for an error at every point it might occur, just add an exception handler to your PL/SQL block. If the exception is ever raised in that block (or any sub-block), you can be sure it will be handled.
Sometimes the error is not immediately obvious, and cannot be detected until later when you perform calculations using bad data. Again, a single exception handler can trap all division-by-zero errors, bad array subscripts, and so on.
If you need to check for errors at a specific spot, you can enclose a single statement or a group of statements inside its own BEGIN-END block with its own exception handler. You can make the checking as general or as precise as you like.
Isolating error-handling routines makes the rest of the program easier to read and understand.
An internal exception is raised automatically if your PL/SQL program violates an Oracle rule or exceeds a system-dependent limit. PL/SQL predefines some common Oracle errors as exceptions. For example, PL/SQL raises the predefined exception NO_DATA_FOUND if a SELECT INTO statement returns no rows.
You can use the pragma EXCEPTION_INIT to associate exception names with other Oracle error codes that you can anticipate. To handle unexpected Oracle errors, you can use the OTHERS handler. Within this handler, you can invoke the functions SQLCODE and SQLERRM to return the Oracle error code and message text. Once you know the error code, you can use it with pragma EXCEPTION_INIT and write a handler specifically for that error.
PL/SQL declares predefined exceptions globally in package STANDARD. You need not declare them yourself. You can write handlers for predefined exceptions using the names in the following table:
PL/SQL lets you define exceptions of your own. Unlike predefined exceptions, user-defined exceptions must be declared and must be raised explicitly by RAISE statements.
Topics:
Associating a PL/SQL Exception with a Number (Pragma EXCEPTION_INIT)
Defining Your Own Error Messages (RAISE_APPLICATION_ERROR Procedure)
Exceptions can be declared only in the declarative part of a PL/SQL block, subprogram, or package. You declare an exception by introducing its name, followed by the keyword EXCEPTION. In the following example, you declare an exception named past_due:
DECLARE past_due EXCEPTION;
Exception and variable declarations are similar. But remember, an exception is an error condition, not a data item. Unlike variables, exceptions cannot appear in assignment statements or SQL statements. However, the same scope rules apply to variables and exceptions.
You cannot declare an exception twice in the same block. You can, however, declare the same exception in two different blocks.
Exceptions declared in a block are considered local to that block and global to all its sub-blocks. Because a block can reference only local or global exceptions, enclosing blocks cannot reference exceptions declared in a sub-block.
If you redeclare a global exception in a sub-block, the local declaration prevails. The sub-block cannot reference the global exception, unless the exception is declared in a labeled block and you qualify its name with the block label block_label.exception_name.
Example 11-3 illustrates the scope rules:
Example 11-3 Scope of PL/SQL Exceptions
DECLARE
past_due EXCEPTION;
acct_num NUMBER;
BEGIN
DECLARE ---------- sub-block begins
past_due EXCEPTION; -- this declaration prevails
acct_num NUMBER;
due_date DATE := SYSDATE - 1;
todays_date DATE := SYSDATE;
BEGIN
IF due_date < todays_date THEN
RAISE past_due; -- this is not handled
END IF;
END; ------------- sub-block ends
EXCEPTION
-- Does not handle raised exception
WHEN past_due THEN
DBMS_OUTPUT.PUT_LINE
('Handling PAST_DUE exception.');
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE
('Could not recognize PAST_DUE_EXCEPTION in this scope.');
END;
/
The enclosing block does not handle the raised exception because the declaration of past_due in the sub-block prevails. Though they share the same name, the two past_due exceptions are different, just as the two acct_num variables share the same name but are different variables. Thus, the RAISE statement and the WHEN clause refer to different exceptions. To have the enclosing block handle the raised exception, you must remove its declaration from the sub-block or define an OTHERS handler.
To handle error conditions (typically ORA- messages) 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.
You code the pragma EXCEPTION_INIT in the declarative part of a PL/SQL block, subprogram, or package using the following 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, as shown in Example 11-4.
The RAISE_APPLICATION_ERROR procedure lets you issue user-defined ORA- error messages from stored subprograms. That way, you can report errors to your application and avoid returning unhandled exceptions.
To invoke RAISE_APPLICATION_ERROR, use the following syntax:
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. RAISE_APPLICATION_ERROR is part of package DBMS_STANDARD, and as with package STANDARD, you do not need to qualify references to it.
An application can invoke raise_application_error only from an executing stored subprogram (or method). When invoked, raise_application_error ends the subprogram and returns a user-defined error number and message to the application. The error number and message can be trapped like any Oracle error.
In Example 11-5, you invoke raise_application_error if an error condition of your choosing happens (in this case, if the current schema owns less than 1000 tables):
Example 11-5 Raising an Application Error with raise_application_error
DECLARE
num_tables NUMBER;
BEGIN
SELECT COUNT(*) INTO num_tables FROM USER_TABLES;
IF num_tables < 1000 THEN
/* Issue your own error code (ORA-20101)
with your own error message. You do not need to
qualify raise_application_error with
DBMS_STANDARD */
raise_application_error
(-20101, 'Expecting at least 1000 tables');
ELSE
-- Do rest of processing (for nonerror case)
NULL;
END IF;
END;
/
The invoking application gets a PL/SQL exception, which it can process using the error-reporting functions SQLCODE and SQLERRM in an OTHERS handler. Also, it can use the pragma EXCEPTION_INIT to map specific error numbers returned by raise_application_error to exceptions of its own, as the following Pro*C example shows:
EXEC SQL EXECUTE
/* Execute embedded PL/SQL block using host
variables v_emp_id and v_amount, which were
assigned values in the host environment. */
DECLARE
null_salary EXCEPTION;
/* Map error number returned by raise_application_error
to user-defined exception. */
PRAGMA EXCEPTION_INIT(null_salary, -20101);
BEGIN
raise_salary(:v_emp_id, :v_amount);
EXCEPTION
WHEN null_salary THEN
INSERT INTO emp_audit VALUES (:v_emp_id, ...);
END;
END-EXEC;
This technique allows the invoking application to handle error conditions in specific exception handlers.
Remember, PL/SQL declares predefined exceptions globally in package STANDARD, so you need not declare them yourself. Redeclaring predefined exceptions is error prone because your local declaration overrides the global declaration. For example, if you declare an exception named invalid_number and then PL/SQL raises the predefined exception INVALID_NUMBER internally, a handler written for INVALID_NUMBER will not catch the internal exception. In such cases, you must use dot notation to specify the predefined exception, as follows:
EXCEPTION
WHEN invalid_number OR STANDARD.INVALID_NUMBER THEN
-- handle the error
END;
Internal exceptions are raised implicitly by the run-time system, as are user-defined exceptions that you have associated with an Oracle error number using EXCEPTION_INIT. However, other user-defined exceptions must be raised explicitly by RAISE statements.
Raise an exception in a PL/SQL block or subprogram only when an error makes it undesirable or impossible to finish processing. You can place RAISE statements for a given exception anywhere within the scope of that exception. In Example 11-6, you alert your PL/SQL block to a user-defined exception named out_of_stock.
Example 11-6 Using RAISE to Force a User-Defined Exception
DECLARE
out_of_stock EXCEPTION;
number_on_hand NUMBER := 0;
BEGIN
IF number_on_hand < 1 THEN
RAISE out_of_stock; -- raise an exception that we defined
END IF;
EXCEPTION
WHEN out_of_stock THEN
-- handle the error
DBMS_OUTPUT.PUT_LINE('Encountered out-of-stock error.');
END;
/
You can also raise a predefined exception explicitly. That way, an exception handler written for the predefined exception can process other errors, as Example 11-7 shows:
Example 11-7 Using RAISE to Force a Pre-Defined Exception
DECLARE
acct_type INTEGER := 7;
BEGIN
IF acct_type NOT IN (1, 2, 3) THEN
RAISE INVALID_NUMBER; -- raise predefined exception
END IF;
EXCEPTION
WHEN INVALID_NUMBER THEN
DBMS_OUTPUT.PUT_LINE
('HANDLING INVALID INPUT BY ROLLING BACK.');
ROLLBACK;
END;
/
When an exception is raised, if PL/SQL cannot find a handler for it in the current block or subprogram, the exception propagates. That is, the exception reproduces itself in successive enclosing blocks until a handler is found or there are no more blocks to search. If no handler is found, PL/SQL returns an unhandled exception error to the host environment.
Exceptions cannot propagate across remote subprogram calls done through database links. A PL/SQL block cannot catch an exception raised by a remote subprogram. For a workaround, see "Defining Your Own Error Messages (RAISE_APPLICATION_ERROR Procedure)".
Figure 11-1, Figure 11-2, and Figure 11-3 illustrate the basic propagation rules.
An exception can propagate beyond its scope, that is, beyond the block in which it was declared, as shown in Example 11-8.
Example 11-8 Scope of an Exception
BEGIN
DECLARE ---------- sub-block begins
past_due EXCEPTION;
due_date DATE := trunc(SYSDATE) - 1;
todays_date DATE := trunc(SYSDATE);
BEGIN
IF due_date < todays_date THEN
RAISE past_due;
END IF;
END; ------------- sub-block ends
EXCEPTION
WHEN OTHERS THEN
ROLLBACK;
END;
/
Because the block that declares the exception past_due has no handler for it, the exception propagates to the enclosing block. But the enclosing block cannot reference the name PAST_DUE, because the scope where it was declared no longer exists. Once the exception name is lost, only an OTHERS handler can catch the exception. If there is no handler for a user-defined exception, the invoking application gets this error:
ORA-06510: PL/SQL: unhandled user-defined exception
Sometimes, you want to reraise an exception, that is, handle it locally, then pass it to an enclosing block. For example, you might want to roll back a transaction in the current block, then log the error in an enclosing block.
To reraise an exception, use a RAISE statement without an exception name, which is allowed only in an exception handler:
Example 11-9 Reraising a PL/SQL Exception
DECLARE
salary_too_high EXCEPTION;
current_salary NUMBER := 20000;
max_salary NUMBER := 10000;
erroneous_salary NUMBER;
BEGIN
BEGIN ---------- sub-block begins
IF current_salary > max_salary THEN
RAISE salary_too_high; -- raise the exception
END IF;
EXCEPTION
WHEN salary_too_high THEN
-- first step in handling the error
DBMS_OUTPUT.PUT_LINE('Salary ' || erroneous_salary ||
' is out of range.');
DBMS_OUTPUT.PUT_LINE
('Maximum salary is ' || max_salary || '.');
RAISE; -- reraise the current exception
END; ------------ sub-block ends
EXCEPTION
WHEN salary_too_high THEN
-- handle the error more thoroughly
erroneous_salary := current_salary;
current_salary := max_salary;
DBMS_OUTPUT.PUT_LINE('Revising salary from ' || erroneous_salary ||
' to ' || current_salary || '.');
END;
/
When an exception is raised, normal execution of your PL/SQL block or subprogram stops and control transfers to its exception-handling part, which is formatted as follows:
EXCEPTION
WHEN exception1 THEN -- handler for exception1
sequence_of_statements1
WHEN exception2 THEN -- another handler for exception2
sequence_of_statements2
...
WHEN OTHERS THEN -- optional handler for all other errors
sequence_of_statements3
END;
To catch raised exceptions, you write exception handlers. Each handler consists of a WHEN clause, which specifies an exception, followed by a sequence of statements to be executed when that exception is raised. These statements complete execution of the block or subprogram; control does not return to where the exception was raised. In other words, you cannot resume processing where you left off.
The optional OTHERS exception handler, which is always the last handler in a block or subprogram, acts as the handler for all exceptions not named specifically. Thus, a block or subprogram can have only one OTHERS handler. Use of the OTHERS handler guarantees that no exception will go unhandled.
If you want two or more exceptions to execute the same sequence of statements, list the exception names in the WHEN clause, separating them by the keyword OR, as follows:
EXCEPTION
WHEN over_limit OR under_limit OR VALUE_ERROR THEN
-- handle the error
If any of the exceptions in the list is raised, the associated sequence of statements is executed. The keyword OTHERS cannot appear in the list of exception names; it must appear by itself. You can have any number of exception handlers, and each handler can associate a list of exceptions with a sequence of statements. However, an exception name can appear only once in the exception-handling part of a PL/SQL block or subprogram.
The usual scoping rules for PL/SQL variables apply, so you can reference local and global variables in an exception handler. However, when an exception is raised inside a cursor FOR loop, the cursor is closed implicitly before the handler is invoked. Therefore, the values of explicit cursor attributes are not available in the handler.
Topics:
Exceptions can be raised in declarations by faulty initialization expressions. For example, the following declaration raises an exception because the constant credit_limit cannot store numbers larger than 999:
Example 11-10 Raising an Exception in a Declaration
DECLARE
-- Raises an error:
credit_limit CONSTANT NUMBER(3) := 5000;
BEGIN
NULL;
EXCEPTION
WHEN OTHERS THEN
-- Cannot catch exception. This handler is never invoked.
DBMS_OUTPUT.PUT_LINE
('Can''t handle an exception in a declaration.');
END;
/
Handlers in the current block cannot catch the raised exception because an exception raised in a declaration propagates immediately to the enclosing block.
When an exception occurs within an exception handler, that same handler cannot catch the exception. An exception raised inside a handler propagates immediately to the enclosing block, which is searched to find a handler for this new exception. From there on, the exception propagates normally. For example:
EXCEPTION
WHEN INVALID_NUMBER THEN
INSERT INTO ... -- might raise DUP_VAL_ON_INDEX
WHEN DUP_VAL_ON_INDEX THEN -- cannot catch exception
END;
A GOTO statement can branch from an exception handler into an enclosing block.
A GOTO statement cannot branch into an exception handler, or from an exception handler into the current block.
In an exception handler, you can use the built-in functions SQLCODE and SQLERRM to find out which error occurred and to get the associated error message. For internal exceptions, SQLCODE returns the number of the Oracle error. The number that SQLCODE returns is negative unless the Oracle error is no data found, in which case SQLCODE returns +100. SQLERRM returns the corresponding error message. The message begins with the Oracle error code.
For user-defined exceptions, SQLCODE returns +1 and SQLERRM returns the message User-Defined Exception unless you used the pragma EXCEPTION_INIT to associate the exception name with an Oracle error number, in which case SQLCODE returns that error number and SQLERRM returns the corresponding error message. The maximum length of an Oracle error message is 512 characters including the error code, nested messages, and message inserts such as table and column names.
If no exception was raised, SQLCODE returns zero and SQLERRM returns the following message:
ORA-0000: normal, successful completion
You can pass an error number to SQLERRM, in which case SQLERRM returns the message associated with that error number. Make sure you pass negative error numbers to SQLERRM.
Passing a positive number to SQLERRM always returns the message user-defined exception unless you pass +100, in which case SQLERRM returns the message no data found. Passing a zero to SQLERRM always returns the following message:
ORA-0000: normal, successful completion
You cannot use SQLCODE or SQLERRM directly in a SQL statement. Instead, you must assign their values to local variables, then use the variables in the SQL statement, as shown in Example 11-11.
Example 11-11 Displaying SQLCODE and SQLERRM
CREATE TABLE errors
(code NUMBER, message VARCHAR2(64), happened TIMESTAMP);
DECLARE
name employees.last_name%TYPE;
v_code NUMBER;
v_errm VARCHAR2(64);
BEGIN
SELECT last_name INTO name
FROM employees
WHERE employee_id = -1;
EXCEPTION
WHEN OTHERS THEN
v_code := SQLCODE;
v_errm := SUBSTR(SQLERRM, 1 , 64);
DBMS_OUTPUT.PUT_LINE
('Error code ' || v_code || ': ' || v_errm);
-- Invoke another procedure,
-- declared with PRAGMA AUTONOMOUS_TRANSACTION,
-- to insert information about errors.
INSERT INTO errors VALUES (v_code, v_errm, SYSTIMESTAMP);
END;
/
The string function SUBSTR ensures that a VALUE_ERROR exception (for truncation) is not raised when you assign the value of SQLERRM to err_msg. The functions SQLCODE and SQLERRM are especially useful in the OTHERS exception handler because they tell you which internal exception was raised.
When using pragma RESTRICT_REFERENCES to assert the purity of a stored function, you cannot specify the constraints WNPS and RNPS if the function invokes SQLCODE or SQLERRM.
Remember, if it cannot find a handler for a raised exception, PL/SQL returns an unhandled exception error to the host environment, which determines the outcome. For example, in the Oracle Precompilers environment, any database changes made by a failed SQL statement or PL/SQL block are rolled back.
Unhandled exceptions can also affect subprograms. If you exit a subprogram successfully, PL/SQL assigns values to OUT parameters. However, if you exit with an unhandled exception, PL/SQL does not assign values to OUT parameters (unless they are NOCOPY parameters). Also, if a stored subprogram fails with an unhandled exception, PL/SQL does not roll back database work done by the subprogram.
You can avoid unhandled exceptions by coding an OTHERS handler at the topmost level of every PL/SQL program.
An exception handler lets you recover from an otherwise fatal error before exiting a block. But when the handler completes, the block is terminated. You cannot return to the current block from an exception handler. In the following example, if the SELECT INTO statement raises ZERO_DIVIDE, you cannot resume with the INSERT statement:
CREATE TABLE employees_temp AS
SELECT employee_id, salary,
commission_pct FROM employees;
DECLARE
sal_calc NUMBER(8,2);
BEGIN
INSERT INTO employees_temp VALUES (301, 2500, 0);
SELECT salary / commission_pct INTO sal_calc
FROM employees_temp
WHERE employee_id = 301;
INSERT INTO employees_temp VALUES (302, sal_calc/100, .1);
EXCEPTION
WHEN ZERO_DIVIDE THEN
NULL;
END;
/
You can still handle an exception for a statement, then continue with the next statement. Place the statement in its own sub-block with its own exception handlers. If an error occurs in the sub-block, a local handler can catch the exception. When the sub-block ends, the enclosing block continues to execute at the point where the sub-block ends, as shown in Example 11-12.
Example 11-12 Continuing After an Exception
DECLARE
sal_calc NUMBER(8,2);
BEGIN
INSERT INTO employees_temp VALUES (303, 2500, 0);
BEGIN -- sub-block begins
SELECT salary / commission_pct INTO sal_calc
FROM employees_temp
WHERE employee_id = 301;
EXCEPTION
WHEN ZERO_DIVIDE THEN
sal_calc := 2500;
END; -- sub-block ends
INSERT INTO employees_temp VALUES (304, sal_calc/100, .1);
EXCEPTION
WHEN ZERO_DIVIDE THEN
NULL;
END;
/
In this example, if the SELECT INTO statement raises a ZERO_DIVIDE exception, the local handler catches it and sets sal_calc to 2500. Execution of the handler is complete, so the sub-block terminates, and execution continues with the INSERT statement.
See Also:
Example 5-38, "Collection Exceptions"You can also perform a sequence of DML operations where some might fail, and process the exceptions only after the entire operation is complete, as described in "Handling FORALL Exceptions (%BULK_EXCEPTIONS Attribute)".
After an exception is raised, rather than abandon your transaction, you might want to retry it. The technique is:
Encase the transaction in a sub-block.
Place the sub-block inside a loop that repeats the transaction.
Before starting the transaction, mark a savepoint. If the transaction succeeds, commit, then exit from the loop. If the transaction fails, control transfers to the exception handler, where you roll back to the savepoint undoing any changes, then try to fix the problem.
In Example 11-13, the INSERT statement might raise an exception because of a duplicate value in a unique column. In that case, we change the value that needs to be unique and continue with the next loop iteration. If the INSERT succeeds, we exit from the loop immediately. With this technique, use a FOR or WHILE loop to limit the number of attempts.
Example 11-13 Retrying a Transaction After an Exception
CREATE TABLE results (res_name VARCHAR(20), res_answer VARCHAR2(3));
CREATE UNIQUE INDEX res_name_ix ON results (res_name);
INSERT INTO results VALUES ('SMYTHE', 'YES');
INSERT INTO results VALUES ('JONES', 'NO');
DECLARE
name VARCHAR2(20) := 'SMYTHE';
answer VARCHAR2(3) := 'NO';
suffix NUMBER := 1;
BEGIN
FOR i IN 1..5 LOOP -- try 5 times
BEGIN -- sub-block begins
SAVEPOINT start_transaction; -- mark a savepoint
/* Remove rows from a table of survey results. */
DELETE FROM results WHERE res_answer = 'NO';
/* Add a survey respondent's name and answers. */
INSERT INTO results VALUES (name, answer);
-- raises DUP_VAL_ON_INDEX
-- if two respondents have the same name
COMMIT;
EXIT;
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
ROLLBACK TO start_transaction; -- undo changes
suffix := suffix + 1; -- try to fix problem
name := name || TO_CHAR(suffix);
END; -- sub-block ends
END LOOP;
END;
/
Using one exception handler for a sequence of statements, such as INSERT, DELETE, or UPDATE statements, can mask the statement that caused an error. If you need to know which statement failed, you can use a locator variable, as in Example 11-14.
Example 11-14 Using a Locator Variable to Identify the Location of an Exception
CREATE OR REPLACE PROCEDURE loc_var AS
stmt_no NUMBER;
name VARCHAR2(100);
BEGIN
stmt_no := 1; -- designates 1st SELECT statement
SELECT table_name INTO name
FROM user_tables
WHERE table_name LIKE 'ABC%';
stmt_no := 2; -- designates 2nd SELECT statement
SELECT table_name INTO name
FROM user_tables
WHERE table_name LIKE 'XYZ%';
EXCEPTION
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE
('Table name not found in query ' || stmt_no);
END;
/
CALL loc_var();
To make your programs more robust and avoid problems at run time, you can turn on checking for certain warning conditions. These conditions are not serious enough to produce an error and keep you from compiling a subprogram. They might point out something in the subprogram that produces an undefined result or might create a performance problem.
To work with PL/SQL warning messages, you use the PLSQL_WARNINGS initialization parameter, the DBMS_WARNING package, and the static data dictionary views *_PLSQL_OBJECT_SETTINGS.
Topics:
PL/SQL warning messages are divided into the categories listed and described in Table 11-1. You can suppress or display groups of similar warnings during compilation. To refer to all warning messages, use the keyword All.
Table 11-1 PL/SQL Warning Categories
| Category | Description | Example |
|---|---|---|
|
|
Condition might cause unexpected action or wrong results. |
Aliasing problems with parameters |
|
|
Condition might cause performance problems. |
Passing a |
|
|
Condition does not affect performance or correctness, but you might want to change it to make the code more maintainable. |
Code that can never be executed |
You can also treat particular messages as errors instead of warnings. For example, if you know that the warning message PLW-05003 represents a serious problem in your code, including 'ERROR:05003' in the PLSQL_WARNINGS setting makes that condition trigger an error message (PLS_05003) instead of a warning message. An error message causes the compilation to fail.
To let the database issue warning messages during PL/SQL compilation, you set the initialization parameter PLSQL_WARNINGS. You can enable and disable entire categories of warnings (ALL, SEVERE, INFORMATIONAL, PERFORMANCE), enable and disable specific message numbers, and make the database treat certain warnings as compilation errors so that those conditions must be corrected.
This parameter can be set at the system level or the session level. You can also set it for a single compilation by including it as part of an ALTER COMPILE statement. You might turn on all warnings during development, turn off all warnings when deploying for production, or turn on some warnings when working on a particular subprogram where you are concerned with some aspect, such as unnecessary code or performance.
Example 11-15 Controlling the Display of PL/SQL Warnings
-- Focus on one aspect: ALTER SESSION SET PLSQL_WARNINGS='ENABLE:PERFORMANCE'; -- Recompile with extra checking: ALTER PROCEDURE loc_var COMPILE PLSQL_WARNINGS='ENABLE:PERFORMANCE' REUSE SETTINGS; -- Turn off warnings: ALTER SESSION SET PLSQL_WARNINGS='DISABLE:ALL'; -- Display 'severe' warnings but not 'performance' warnings, -- display PLW-06002 warnings to produce errors that halt compilation: ALTER SESSION SET PLSQL_WARNINGS='ENABLE:SEVERE', 'DISABLE:PERFORMANCE', 'ERROR:06002'; -- For debugging during development ALTER SESSION SET PLSQL_WARNINGS='ENABLE:ALL';
Warning messages can be issued during compilation of PL/SQL subprograms; anonymous blocks do not produce any warnings.
To see any warnings generated during compilation, use the SQL*Plus SHOW ERRORS statement or query the static data dictionary view USER_ERRORS. PL/SQL warning messages use the prefix PLW.
For general information about PL/SQL initialization parameters, see "Initialization Parameters for PL/SQL Compilation".
If you are writing PL/SQL subprograms in a development environment that compiles them, you can control PL/SQL warning messages by invoking subprograms in the DBMS_WARNING package. You can also use this package when compiling a complex application, made up of several nested SQL*Plus scripts, where different warning settings apply to different subprograms. You can save the current state of the PLSQL_WARNINGS parameter with one call to the package, change the parameter to compile a particular set of subprograms, then restore the original parameter value.
The procedure in Example 11-16 has unnecessary code that can be removed. It could represent a mistake, or it could be intentionally hidden by a debug flag, so you might or might not want a warning message for it.
Example 11-16 Using the DBMS_WARNING Package to Display Warnings
-- When warnings disabled,
-- the following procedure compiles with no warnings
CREATE OR REPLACE PROCEDURE unreachable_code AS
x CONSTANT BOOLEAN := TRUE;
BEGIN
IF x THEN
DBMS_OUTPUT.PUT_LINE('TRUE');
ELSE
DBMS_OUTPUT.PUT_LINE('FALSE');
END IF;
END unreachable_code;
/
-- enable all warning messages for this session
CALL DBMS_WARNING.set_warning_setting_string
('ENABLE:ALL' ,'SESSION');
-- Check the current warning setting
SELECT DBMS_WARNING.get_warning_setting_string() FROM DUAL;
-- Recompile procedure
-- and warning about unreachable code displays
ALTER PROCEDURE unreachable_code COMPILE;
SHOW ERRORS;
In Example 11-16, you could have used the following ALTER PROCEDURE without invoking DBMS_WARNINGS.set_warning_setting_string:
ALTER PROCEDURE unreachable_code COMPILE PLSQL_WARNINGS = 'ENABLE:ALL' REUSE SETTINGS;
For more information, see ALTER PROCEDURE in Oracle Database SQL Language Reference, DBMS_WARNING package in Oracle Database PL/SQL Packages and Types Reference, and PLW- messages in Oracle Database Error Messages
