PL/SQL User's Guide and Reference

Overview

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.