What should be considered when using the rownum function in SQL?
When using the ROWNUM function in SQL, it is important to keep the following points in mind:
- ROWNUM is a pseudo column unique to Oracle databases that is used to identify the sequence number of rows returned in a result set, which is assigned only after the result set is returned.
- ROWNUM is assigned after the query result is returned, so it cannot be directly used for filtering conditions in the WHERE clause.
- To use ROWNUM for filtering conditions in query results, wrap ROWNUM in a subquery and then use it in the outer query.
- Be cautious when using ROWNUM in the ORDER BY clause, as ROWNUM is assigned after the query results are returned, making it unreliable to use ROWNUM before sorting.
- When using ROWNUM, make sure the query results are ordered to avoid unexpected outcomes because ROWNUM is assigned after the query results are returned.
- In Oracle 12c and later versions, the replacement function for ROWNUM is ROW_NUMBER() OVER(), which offers more flexibility and better performance.