Nov 27, 2014

ROWID & ROWNUM Pseudocolumn

What is ROWID Pseudocolumns ::
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:
  • The data object number of the object
  • The data block in the datafile in which the row resides
  • The position of the row in the data block (first row is 0)
  • 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:
  • 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.

Don't use ROW_ID with MIN and MAX as MAX(ROWID) won't  always return me last inserted row because that row should not  have the biggest ROWID value.
The value of ROW_ID is not in sequence but it's only unique value.

What is ROWNUM Pseudocolumns ::
For each row returned by a query, the ROWNUM pseudocolumn returns a number indicating the order in which Oracle selects the row from a table or set of joined rows. The first row selected has a ROWNUM of 1, the second has 2, and so on.
You can use ROWNUM to limit the number of rows returned by a query, as in this example:
SELECT * FROM employees WHERE ROWNUM < 10; 

If an ORDER BY clause follows ROWNUM in the same query, then the rows will be reordered by the ORDER BY clause. The results can vary depending on the way the rows are accessed. For example, if the ORDER BY clause causes Oracle to use an index to access the data, then Oracle may retrieve the rows in a different order than without the index. Therefore, the following statement will not have the same effect as the preceding example:
SELECT * FROM employees WHERE ROWNUM < 11 ORDER BY last_name;

If you embed the ORDER BY clause in a subquery and place the ROWNUM condition in the top-level query, then you can force theROWNUM condition to be applied after the ordering of the rows. For example, the following query returns the employees with the 10 smallest employee numbers. This is sometimes referred to as top-N reporting:
   (SELECT * FROM employees ORDER BY employee_id)

In the preceding example, the ROWNUM values are those of the top-level SELECT statement, so they are generated after the rows have already been ordered by employee_id in the subquery.
Conditions testing for ROWNUM values greater than a positive integer are always false. For example, this query returns no rows:
SELECT * FROM employees

The first row fetched is assigned a ROWNUM of 1 and makes the condition false. The second row to be fetched is now the first row and is also assigned a ROWNUM of 1 and makes the condition false. All rows subsequently fail to satisfy the condition, so no rows are returned.
You can also use ROWNUM to assign unique values to each row of a table, as in this example:
UPDATE my_table
    SET column1 = ROWNUM;

Please refer to the function ROW_NUMBER for an alternative method of assigning unique numbers to rows.

Sources ::

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