Просить объяснения этого кода PL / SQL

Я просмотрел и прокомментировал блок. В основном ваш код тестирует настраиваемую обработку ошибок.

DECLARE
  --Define the custom exception
  table_doesnot_exists   EXCEPTION;
  /*
   *In order to use the custom exception we have to declare it (typically used with ORA messages
   *that have no predefined name. Syntax: PRAGMA EXCEPTION_INIT (exception_name, -ORA_ERR_#);
   *In this case: 00942, 00000, "table or view does not exist"
   *Original code cited 942 but the oerr code is actually 00942, the leading 0's in this case are irrelevant
   */
  PRAGMA EXCEPTION_INIT (table_doesnot_exists, -942);
BEGIN
  --Attempt to drop the table
  EXECUTE IMMEDIATE 'DROP TABLE test_table_does_not_exist';
EXCEPTION
  --If the table we attempted does not exist then our custom exception will catch the ORA-00942
  WHEN table_doesnot_exists
  --Now lets throw out a debug output line
  THEN DBMS_OUTPUT.PUT_LINE('table does not exist');
END;
/