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;
BY clause follows
ROWNUM in the same query, then the rows will be reordered by the
BY clause. The results can vary depending on the way the rows are accessed. For example, if the
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
BY clause in a subquery and place the
ROWNUM condition in the top-level query, then you can force the
ROWNUM 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 (SELECT * FROM employees ORDER BY employee_id) WHERE ROWNUM < 11;
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 WHERE ROWNUM > 1;
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.
UPDATE my_table SET column1 = ROWNUM;
Please refer to the function ROW_NUMBER for an alternative method of assigning unique numbers to rows.
ROWNUMin a query can affect view optimization. For more information, see Oracle Database Concepts.