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.
This chapter discusses the following topics:
In PL/SQL, a warning or 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.
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:
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 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 codes.
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 in the following list: