Exception handling
In the PL/SQL, a warning or error condition is known as an exception. The Exceptions can be internally defined (by the run-time system) or user defined. The Examples of internally defined exceptions involve division by zero and out of memory. Some familiar internal exceptions have predefined names, like ZERO_DIVIDE and STORAGE_ERROR.
You can define exceptions of your own in the declarative part of any PL/SQL subprogram, block, or package. For illustration, you might define an exception namely the insufficient_funds to flag overdrawn bank accounts. Dissimilar internal exceptions, user-defined exceptions should be given names.
Whenever errors occur, an exception is raised. That is, the normal execution stops and control transfers to the exception-handling section of your PL/SQL subprogram or block. The Internal exceptions are raised implicitly (automatically) by the run-time system. The User-defined exceptions should be raised explicitly by the RAISE statements that can also raise the predefined exceptions.
To handle the raised exceptions, you write individual routines known as the exception handlers.
Later an exception handler runs, the present block stops executing and the enclosing block resumes with the next statement. If there is no enclosing block, the control returns to the host atmosphere.
In the illustration below, you compute and store a price-to-earnings ratio for a company with ticker symbol XYZ. The predefined exception ZERO_DIVIDE is raised whenever the company has zero earnings. This stops general execution of the block and transfers control to the exception handlers. The elective OTHERS handler catches all the exceptions which the block does not name explicitly.
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 illustration describes an exception handling, which is not the effective use of INSERT statements. For illustration, an enhanced way to do the insert is as shown:
INSERT INTO stats (symbol, ratio)
SELECT symbol, DECODE(earnings, 0, NULL, price / earnings)
FROM stocks WHERE symbol = 'XYZ';