Release 8.0
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| PL/SQL User's Guide and Reference Release 8.0 A54654_01 |
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There is nothing more exhilarating than to be shot at without result.
Winston Churchill
Runtime 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 runtime 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.
Overview
Advantages of Exceptions
Predefined Exceptions
User-Defined Exceptions
How Exceptions Are Raised
How Exceptions Propagate
Handling Raised Exceptions
Useful Techniques
In PL/SQL, a warning or error condition is called an exception. Exceptions can be internally defined (by the runtime 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 runtime 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.
In the example below, you calculate and store a price-to-earnings ratio for a company with ticker symbol XYZ. If the company has zero earnings, the predefined exception ZERO_DIVIDE is raised. This stops normal execution of the block and transfers control to the exception handlers. The optional OTHERS handler catches all exceptions that the block does not name specifically.
DECLARE
pe_ratio NUMBER(3,1);
BEGIN
SELECT price / earnings INTO pe_ratio FROM stocks
WHERE symbol = 'XYZ'; -- might cause division-by-zero error INSERT INTO stats (symbol, ratio) VALUES ('XYZ', pe_ratio);
COMMIT;
EXCEPTION -- exception handlers begin
WHEN ZERO_DIVIDE THEN -- handles 'division by zero' error
INSERT INTO stats (symbol, ratio) VALUES ('XYZ', NULL);
COMMIT;
...
WHEN OTHERS THEN -- handles all other errors
ROLLBACK;
END; -- exception handlers and block end here
The last example illustrates exception handling, not the effective use of INSERT statements. For example, a better way to do the insert follows:
INSERT INTO stats (symbol, ratio)
SELECT symbol, DECODE(earnings, 0, NULL, price / earnings)
FROM stocks WHERE symbol = 'XYZ';
In this example, a subquery supplies values to the INSERT statement. If earnings are zero, the function DECODE returns a null. Otherwise, DECODE returns the price-to-earnings ratio.
Using exceptions for error handling has several advantages. Without exception handling, every time you issue a command, you must check for execution errors, as follows:
BEGIN
SELECT ...
-- check for 'no data found' error
SELECT ...
-- check for 'no data found' error
SELECT ...
-- check for 'no data found' error
Error processing is not clearly separated from normal processing; nor is it robust. If you neglect to code a check, the error goes undetected and is likely to cause other, seemingly unrelated errors.
With exceptions, you can handle errors conveniently without the need to code multiple checks, as follows:
BEGIN
SELECT ...
SELECT ...
SELECT ...
...
EXCEPTION
WHEN NO_DATA_FOUND THEN -- catches all 'no data found' errors
Exceptions improve readability by letting you isolate error-handling routines. The primary algorithm is not obscured by error recovery algorithms. Exceptions also improve reliability. You need not worry about 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.
An internal exception is raised implicitly whenever your PL/SQL program violates an Oracle rule or exceeds a system-dependent limit. Every Oracle error has a number, but exceptions must be handled by name. So, 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.
To handle other Oracle errors, you can use the OTHERS handler. The error-reporting functions SQLCODE and SQLERRM are especially useful in the OTHERS handler because they return the Oracle error code and message text. Alternatively, you can use the pragma EXCEPTION_INIT to associate exception names with Oracle error numbers. (See "Using EXCEPTION_INIT" on page 6-8.)
PL/SQL declares predefined exceptions globally in package STANDARD, which defines the PL/SQL environment. So, you need not declare them yourself. You can write handlers for predefined exceptions using the names shown in the list below. Also shown are the corresponding Oracle error codes and SQLCODE return values.
For a complete list of the messages that Oracle or PL/SQL might issue, see Oracle8 Server Messages.
Brief descriptions of the predefined exceptions follow:
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.
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. So, the sub-block cannot reference the global exception unless it was declared in a labeled block, in which case the following syntax is valid:
block_label.exception_name
The next example illustrates the scope rules:
DECLARE
past_due EXCEPTION;
acct_num NUMBER;
BEGIN
...
DECLARE ---------- sub-block begins
past_due EXCEPTION; -- this declaration prevails
acct_num NUMBER;
BEGIN
...
IF ... THEN
RAISE past_due; -- this is not handled
END IF;
...
END; ------------- sub-block ends
EXCEPTION
WHEN past_due THEN -- does not handle RAISEd exception
...
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. Therefore, 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 unnamed internal exceptions, you must use the OTHERS handler or the pragma EXCEPTION_INIT. A pragma is a compiler directive, which can be thought of as a parenthetical remark to the compiler. Pragmas (also called pseudoinstructions) are processed at compile time, not at run time. For example, in the language Ada, the following pragma tells the compiler to optimize the use of storage space:
pragma OPTIMIZE(SPACE);
In PL/SQL, the pragma EXCEPTION_INIT tells the compiler to associate an exception name with an Oracle error number. That allows you to refer to any internal exception by name and to write a specific handler for it.
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. The pragma must appear somewhere after the exception declaration in the same declarative part, as shown in the following example:
DECLARE
deadlock_detected EXCEPTION;
PRAGMA EXCEPTION_INIT(deadlock_detected, -60);
BEGIN
...
EXCEPTION
WHEN deadlock_detected THEN
-- handle the error
...
END;
Package DBMS_STANDARD, which is supplied with Oracle, provides language facilities that help your application interact with Oracle. For example, the procedure raise_application_error lets you issue user-defined error messages from stored subprograms. That way, you can report errors to your application and avoid returning unhandled exceptions.
To call raise_application_error, you use the 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. Package DBMS_STANDARD is an extension of package STANDARD, so you need not qualify references to it.
An application can call raise_application_error only from an executing stored subprogram. When called, 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 the following example, you call raise_application_error if an employee's salary is missing:
CREATE PROCEDURE raise_salary (emp_id NUMBER, increase NUMBER) AS
current_salary NUMBER;
BEGIN
SELECT sal INTO current_salary FROM emp
WHERE empno = emp_id;
IF current_salary IS NULL THEN
/* Issue user-defined error message. */
raise_application_error(-20101, 'Salary is missing');
ELSE
UPDATE emp SET sal = current_salary + increase
WHERE empno = emp_id;
END IF;
END raise_salary;
The calling 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 follows:
EXEC SQL EXECUTE
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(:emp_number, :amount);
EXCEPTION
WHEN null_salary THEN
INSERT INTO emp_audit VALUES (:emp_number, ...);
...
END;
END-EXEC;
This technique allows the calling 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.
EXCEPTION
WHEN invalid_number OR STANDARD.INVALID_NUMBER THEN
-- handle the error
...
WHEN OTHERS THEN ...
END;
Internal exceptions are raised implicitly by the runtime 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.
PL/SQL blocks and subprograms should raise an exception 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 the following example, you alert your PL/SQL block to a user-defined exception named out_of_stock:
DECLARE
out_of_stock EXCEPTION;
number_on_hand NUMBER(4);
BEGIN
...
IF number_on_hand < 1 THEN
RAISE out_of_stock;
END IF;
...
EXCEPTION
WHEN out_of_stock THEN
-- handle the 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 the following example shows:
DECLARE
acct_type INTEGER;
...
BEGIN
...
IF acct_type NOT IN (1, 2, 3) THEN
RAISE INVALID_NUMBER; -- raise predefined exception
END IF;
...
EXCEPTION
WHEN INVALID_NUMBER THEN
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. In the latter case, PL/SQL returns an unhandled exception error to the host environment.
However, exceptions cannot propagate across remote procedure calls (RPCs). Therefore, a PL/SQL block cannot catch an exception raised by a remote subprogram. For a workaround, see "Using raise_application_error" on page 6-8.
Figure 6-1, Figure 6-2, and Figure 6-3 illustrate the basic propagation rules.
An exception can propagate beyond its scope, that is, beyond the block in which it was declared. Consider the following example:
BEGIN
...
DECLARE ---------- sub-block begins
past_due EXCEPTION;
BEGIN
...
IF ... THEN
RAISE past_due;
END IF;
END; ------------- sub-block ends
EXCEPTION
...
WHEN OTHERS THEN
ROLLBACK;
END;
Because the block in which it was declared has no handler for the exception named past_due, it propagates to the enclosing block. But, according to the scope rules, enclosing blocks cannot reference exceptions declared in a sub-block. So, only an OTHERS handler can catch the 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, simply place a RAISE statement in the local handler, as shown in the following example:
DECLARE
out_of_balance EXCEPTION;
BEGIN
...
BEGIN ---------- sub-block begins
...
IF ... THEN
RAISE out_of_balance; -- raise the exception
END IF; EXCEPTION
WHEN out_of_balance THEN
-- handle the error
RAISE; -- reraise the current exception
...
END; ------------ sub-block ends
EXCEPTION
WHEN out_of_balance THEN
-- handle the error differently
...
END;
Omitting the exception name in a RAISE statement-allowed only in an exception handler-reraises the current exception.
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 exception_name1 THEN -- handler
sequence_of_statements1
WHEN exception_name2 THEN -- another handler
sequence_of_statements2
...
WHEN OTHERS THEN -- optional handler
sequence_of_statements3
To catch raised exceptions, you must 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.
As the following example shows, use of the OTHERS handler guarantees that no exception will go unhandled:
EXCEPTION
WHEN ... THEN
-- handle the error
WHEN ... THEN
-- handle the error
...
WHEN OTHERS THEN
-- handle all other errors
END;
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.
Exceptions can be raised in declarations by faulty initialization expressions. For example, the following declaration raises an exception because the constant limit cannot store numbers larger than 999:
DECLARE
limit CONSTANT NUMBER(3) := 5000; -- raises an exception
BEGIN
...
EXCEPTION
WHEN OTHERS THEN ... -- cannot catch the exception
Handlers in the current block cannot catch the raised exception because an exception raised in a declaration propagates immediately to the enclosing block.
Only one exception at a time can be active in the exception-handling part of a block or subprogram. So, an exception raised inside a handler propagates immediately to the enclosing block, which is searched to find a handler for the newly raised exception. From there on, the exception propagates normally. Consider the following example:
EXCEPTION
WHEN INVALID_NUMBER THEN
INSERT INTO ... -- might raise DUP_VAL_ON_INDEX
WHEN DUP_VAL_ON_INDEX THEN ... -- cannot catch the exception
A GOTO statement cannot branch to an exception handler; nor can it branch from an exception handler into the current block. For example, the following GOTO statement is illegal:
DECLARE
pe_ratio NUMBER(3,1);
BEGIN
DELETE FROM stats WHERE symbol = 'XYZ';
SELECT price / NVL(earnings, 0) INTO pe_ratio FROM stocks
WHERE symbol = 'XYZ';
<<my_label>>
INSERT INTO stats (symbol, ratio) VALUES ('XYZ', pe_ratio);
EXCEPTION
WHEN ZERO_DIVIDE THEN
pe_ratio := 0;
GOTO my_label; -- illegal branch into current block
However, a GOTO statement can branch from an exception handler into an enclosing block.
In an exception handler, you can use the 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 has been raised, SQLCODE returns zero and SQLERRM returns the 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. In the following example, you pass positive numbers and so get unwanted results:
DECLARE
...
err_msg VARCHAR2(100);
BEGIN
...
/* Get all Oracle error messages. */
FOR err_num IN 1..9999 LOOP
err_msg := SQLERRM(err_num); -- wrong; should be -err_num
INSERT INTO errors VALUES (err_msg);
END LOOP;
END;
Passing a positive number to SQLERRM always returns the message
User-Defined Exception
unless you pass +100, in which case SQLERRM returns this message:
ORA-01403: 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 the following example shows:
DECLARE
err_num NUMBER;
err_msg VARCHAR2(100);
BEGIN
...
EXCEPTION
...
WHEN OTHERS THEN
err_num := SQLCODE;
err_msg := SUBSTR(SQLERRM, 1, 100);
INSERT INTO errors VALUES (err_num, err_msg);
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. SQLCODE and SQLERRM are especially useful in the OTHERS exception handler because they tell you which internal exception was raised.
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. 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 block and subprogram.
In this section, you learn three techniques that increase flexibility.
An exception handler lets you recover from an otherwise "fatal" error before exiting a block. But, when the handler completes, the block terminates. 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:
DECLARE
pe_ratio NUMBER(3,1);
BEGIN
DELETE FROM stats WHERE symbol = 'XYZ';
SELECT price / NVL(earnings, 0) INTO pe_ratio FROM stocks
WHERE symbol = 'XYZ';
INSERT INTO stats (symbol, ratio) VALUES ('XYZ', pe_ratio);
EXCEPTION
WHEN ZERO_DIVIDE THEN ...
Though PL/SQL does not support continuable exceptions, you can still handle an exception for a statement, then continue with the next statement. Simply 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 terminates, the enclosing block continues to execute at the point where the sub-block ends. Consider the following example:
DECLARE
pe_ratio NUMBER(3,1);
BEGIN
DELETE FROM stats WHERE symbol = 'XYZ';
BEGIN ---------- sub-block begins
SELECT price / NVL(earnings, 0) INTO pe_ratio FROM stocks
WHERE symbol = 'XYZ';
EXCEPTION
WHEN ZERO_DIVIDE THEN
pe_ratio := 0;
END; ---------- sub-block ends
INSERT INTO stats (symbol, ratio) VALUES ('XYZ', pe_ratio);
EXCEPTION
...
In this example, if the SELECT INTO statement raises a ZERO_DIVIDE exception, the local handler catches it and sets pe_ratio to zero. Execution of the handler is complete, so the sub-block terminates, and execution continues with the INSERT statement.
After an exception is raised, rather than abandon your transaction, you might want to retry it. The technique you use is simple. First, encase the transaction in a sub-block. Then, place the sub-block inside a loop that repeats the transaction.
Before starting the transaction, you mark a savepoint. If the transaction succeeds, you 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.
Consider the example below. When the exception handler completes, the sub-block terminates, control transfers to the LOOP statement in the enclosing block, the sub-block starts executing again, and the transaction is retried. You might want to use a FOR or WHILE loop to limit the number of tries.
DECLARE
name CHAR(20);
ans1 CHAR(3);
ans2 CHAR(3);
ans3 CHAR(3);
suffix NUMBER := 1;
BEGIN
...
LOOP -- could be FOR i IN 1..10 LOOP to allow ten tries
BEGIN -- sub-block begins
SAVEPOINT start_transaction; -- mark a savepoint
/* Remove rows from a table of survey results. */
DELETE FROM results WHERE answer1 = 'NO';
/* Add a survey respondent's name and answers. */
INSERT INTO results VALUES (name, ans1, ans2, ans3);
-- raises DUP_VAL_ON_INDEX if two respondents
-- have the same name (because there is a unique
-- index on the name column)
COMMIT;
EXIT;
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
ROLLBACK TO start_transaction; -- undo changes
suffix := suffix + 1; -- try to fix
name := name || TO_CHAR(suffix); -- problem
...
END; -- sub-block ends
END LOOP;
END;
Exceptions can mask the statement that caused an error, as the following example shows:
BEGIN
SELECT ...
SELECT ...
SELECT ...
...
EXCEPTION
WHEN NO_DATA_FOUND THEN ...
-- Which SELECT statement caused the error?
END;
Normally, this is not a problem. But, if the need arises, you can use a locator variable to track statement execution, as follows:
DECLARE
stmt INTEGER := 1; -- designates 1st SELECT statement
BEGIN
SELECT ...
stmt := 2; -- designates 2nd SELECT statement
SELECT ...
stmt := 3; -- designates 3rd SELECT statement
SELECT ...
...
EXCEPTION
WHEN NO_DATA_FOUND THEN
INSERT INTO errors VALUES ('Error in statement ' || stmt);
...
END;
