PL/SQL User's Guide and Reference

How Exceptions Propagate

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" .

Figure 6 - 1, Figure 6 - 2, and Figure 6 - 3 illustrate the basic propagation rules.

Figure 6 - 1. Propagation Rules: Example 1

Figure 6 - 2. Propagation Rules: Example 2

Figure 6 - 3. Propagation Rules: Example 3

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.