ROWID Pseudocolumn
For each row in the database, the ROWID pseudocolumn returns the address of the row. Oracle Database rowid values contain information necessary to locate a row:
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The data object number of the object
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The data block in the datafile in which the row resides
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The position of the row in the data block (first row is 0)
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The datafile in which the row resides (first file is 1). The file number is relative to the tablespace.
Usually, a rowid value uniquely identifies a row in the database. However, rows in different tables that are stored together in the same cluster can have the same rowid.
Values of the ROWID pseudocolumn have the datatype ROWID or UROWID. Please refer to "ROWID Datatype" and "UROWID Datatype" for more information.
Rowid values have several important uses:
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They are the fastest way to access a single row.
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They can show you how the rows in a table are stored.
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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