How to recover from Errors in Oracle

There are a few ways to recover from errors at the command line in Oracle if you make a mistake. Here are a few tips:

1. Use the `>` operator to redirect output to a file. This way, you can review the output later if you need to troubleshoot the error. For example:

   "
   sqlplus scott/tiger > output.txt
   "
   

2. Use the `SPOOL` command to spool output to a file. This is similar to redirecting output to a file, but it allows you to pause and resume spooling. For example:
   

   "
   sqlplus scott/tiger
   SPOOL output.txt
   SELECT * FROM emp;
   SPOOL OFF
   "
   

3. Use the `ROLLBACK` command to roll back all changes made since the last commit. This can be useful if you make a mistake and need to revert your changes. For example:
   

   "
   sqlplus scott/tiger
   BEGIN
     UPDATE emp SET salary = salary * 1.1;
     COMMIT;
   END;
   /
   ROLLBACK;
   "
   

4. Use the `SHOW ERRORS` command to display detailed error information.
   This can be helpful in diagnosing the cause of an error. For example:
   

   "
   sqlplus scott/tiger
   BEGIN
     UPDATE emp SET salary = salary * 1.1 WHERE empno = 100;
   END;
   /
   SHOW ERRORS;
   "
   

5. Use the `SQLCODE` and `SQLERRM` variables to get more information about an error. The `SQLCODE` variable contains the error code, and the `SQLERRM` variable contains the error message. For example:
   

   "
   sqlplus scott/tiger
   DECLARE
     v_sqlcode NUMBER;
     v_sqlerrm VARCHAR2(200);
   BEGIN
     BEGIN
       UPDATE emp SET salary = salary * 1.1 WHERE empno = 100;
     EXCEPTION
       WHEN OTHERS THEN
         v_sqlcode := SQLCODE;
         v_sqlerrm := SQLERRM;
         RAISE_APPLICATION_ERROR(-20001, v_sqlerrm);
     END;
   END;
   /
   "
   

If you are unable to recover from an error on your own, you may need to consult the Oracle documentation or contact Oracle support for assistance. Here are some additional tips for avoiding errors at the command line in Oracle:
Avoiding errors at the Command Line in Oracle

1. Carefully review your commands before executing them. Make sure that you have the correct syntax and that you are specifying the correct values for all parameters.
2. Use the `EXPLAIN PLAN` command to review the execution plan for a query before executing it. This can help you to identify and avoid performance problems.
3. Use the `SAVEPOINT` command to create savepoints within a transaction. This allows you to roll back to a specific point in time if you make a mistake.
4. Use the `BACKUP` and `RESTORE` commands to back up and restore your database regularly. This will help you to recover from errors quickly and easily.

Here is what is happening in a situation where the command line does not recognize the user input. SQL*Plus is designed to let you enter and execute SQL statements. SQL statements may be many lines long, and SQL*Plus must allow for this. When you type in a command that is not one of the SQL*Plus specific commands, such as help or exit, the program assumes that you are entering an SQL statement. Once SQL*Plus thinks that you are entering an SQL statement, it accepts and buffers all text that you type until you terminate the statement, usually with a semicolon. Only at that point does it try to execute the statement.

The solution to the problem of mistyping a command is to enter a semicolon to terminate the statement, accept the error that occurs, and retry your command. When you do this, SQL*Plus returns the word it does not recognize.

SQL*Plus recognizes when you enter an incorrect command into the command line. It does this by parsing the command and checking it against a list of known commands and syntax rules. If SQL*Plus does not recognize the command or if the command contains invalid syntax, it will display an error message. For example, if you enter the following command:

SELECT * FROM emp WHERE salary > 1000000;

SQL*Plus will display the following error message:

ORA-00904: "SALARY" non-existent column

This error message indicates that SQL*Plus does not recognize the `salary` column in the `emp` table.
In addition to displaying error messages, SQL*Plus also provides a number of features to help you debug errors, such as the `SHOW ERRORS` command and the `SQLCODE` and `SQLERRM` variables. Here are some tips for avoiding errors at the command line in SQL*Plus:

1. Carefully review your commands before executing them.
2. Use the `EXPLAIN PLAN` command to review the execution plan for a query before executing it.
3. Use the `SAVEPOINT` command to create savepoints within a transaction.
4. Use the `BACKUP` and `RESTORE` commands to back up and restore your database regularly.

If you do make a mistake, SQL*Plus will provide you with the information you need to debug and fix the error.