How to Limit Rows in Oracle: Techniques and Best Practices

Discover different ways to limit rows in Oracle using techniques such as the `ROWNUM` and `FETCH` clauses, and understand how they can improve query efficiency and readability.
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In Oracle databases, there are several methods to limit the number of rows returned by a query. This is useful for scenarios such as paginating results, optimizing performance, and enhancing readability of results. Here are some common approaches to limit rows in Oracle:

Using ROWNUM

The ROWNUM pseudo-column in Oracle assigns a unique row number to each row returned by a query, starting from 1 for the first row. You can use ROWNUM to limit the number of rows returned by a query. For example, to retrieve the first 10 rows from a table:

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However, keep in mind that ROWNUM is assigned before sorting, which can lead to unexpected results if you are trying to limit rows after sorting them. To address this, you can use a subquery:

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This way, the inner query sorts the data first, and then the outer query limits the results using ROWNUM.

Using FETCH Clause

In more recent versions of Oracle (12c and later), you can use the FETCH clause to limit the number of rows returned by a query. This is similar to the LIMIT clause in other databases. For example, to retrieve the first 10 rows from a table:

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This approach is more readable and straightforward than using ROWNUM, especially when combined with an ORDER BY clause.

Combining OFFSET and FETCH Clauses

The FETCH clause can also be combined with the OFFSET clause to skip a specified number of rows before fetching a set number of rows. For example, to retrieve rows 11 through 20:

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This combination is useful for implementing pagination in your queries.

Conclusion

Limiting the number of rows returned by a query can improve performance and make your queries easier to read and manage. The ROWNUM and FETCH clauses are both effective ways to achieve this, but using the FETCH clause with an ORDER BY statement is typically the most modern and efficient approach. Choose the method that best suits your specific use case and Oracle version.