In SQL, ROWNUM is a pseudocolumn that is used in Oracle and related databases to assign a unique sequential number to each row returned by a query result set. It is often used for pagination or limiting the number of rows returned in a query.
Syntax
SELECT ROWNUM, column1, column2, ...
FROM table_name
WHERE condition;
The ROWNUM pseudocolumn is typically used in the SELECT statement to retrieve the sequential row number along with other columns from the table.
Example
Consider a table employees:
SELECT ROWNUM, employee_id, first_name, last_name
FROM employees;
This query retrieves the row number along with the employee_id, first_name, and last_name columns from the employees table.
Usage
- Pagination: Limit the number of rows returned by a query, especially when implementing pagination in web applications.
- Row Ordering: Retrieve rows in a specific order and assign a sequential number to each row.
- Data Analysis: Analyze data by assigning a unique identifier to each row for further processing.
Considerations
- Ordering:
ROWNUM is assigned before the ORDER BY clause is applied, so you should use it carefully if ordering is important.
- Filtering: When using
ROWNUM with filtering conditions, ensure that you apply the filter before selecting rows based on ROWNUM to avoid unexpected results.
ROWNUM is a pseudocolumn in SQL used in Oracle databases to assign a unique sequential number to each row returned by a query result set. It is commonly used for pagination, row ordering, and data analysis tasks. Understanding how to use ROWNUM effectively can be valuable for managing and analyzing data in Oracle databases.
