ROWID Pseudocolumn "ROWID Datatype" and "UROWID Datatype" for more information.

Rowid values have several important uses:

• They are the fastest way to access a single row.

• They can show you how the rows in a table are stored.

• They are unique identifiers for rows in a table.

You should not use ROWID as the primary key of a table. If you delete and reinsert a row with the Import and Export utilities, for example, then its rowid may change. If you delete a row, then Oracle may reassign its rowid to a new row inserted later.

Although you can use the ROWID pseudocolumn in the SELECT and WHERE clause of a query, these pseudocolumn values are not actually stored in the database. You cannot insert, update, or delete a value of the ROWID pseudocolumn.

Example This statement selects the address of all rows that contain data for employees in department 20:

SELECT ROWID, last_name  
   FROM employees
   WHERE department_id = 20;


TOM大师对rowid的解释：https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:53140678334596
ORACLE-FAQ：http://www.orafaq.com/wiki/ROWID