FETCH Clause FAQS

1. What is the FETCH clause in Oracle?

The FETCH clause is used to limit the number of rows returned by a query in Oracle SQL. It is often used in conjunction with the OFFSET clause to implement pagination in result sets, enabling you to return a specific subset of rows.

2. Which Oracle version introduced the FETCH clause?

The FETCH clause was introduced in Oracle 12c (12.1). It was not available in earlier versions of Oracle.

3. What is the difference between FETCH FIRST and FETCH NEXT?

• FETCH FIRST retrieves the first n rows from the result set.
• FETCH NEXT is used with OFFSET to fetch a specific set of rows after skipping a given number of rows, often used in pagination.

Example:

FETCH FIRST 10 ROWS ONLY;  -- Retrieves the first 10 rows

OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY;  -- Skips 10 rows and fetches the next 10 rows

4. How does the OFFSET clause work?

The OFFSET clause specifies how many rows should be skipped from the beginning of the result set before starting to fetch rows. It is commonly used in pagination scenarios.

Example:

OFFSET 10 ROWS;  -- Skips the first 10 rows

When used with FETCH, it allows you to retrieve the next "page" of results:

OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;  -- Skips the first 20 rows and fetches the next 10

5. Can I use FETCH without OFFSET?

Yes, you can use FETCH without OFFSET to retrieve the first n rows of a result set. This is typically used when you only need a limited number of rows, like when returning the first 10 rows of a query.

Example:

SELECT * FROM employees ORDER BY hire_date FETCH FIRST 10 ROWS ONLY;

6. Is ORDER BY required when using FETCH or OFFSET?

Yes, the ORDER BY clause is required when using the FETCH or OFFSET clauses. Without ORDER BY, the result set may be returned in an unpredictable order, which defeats the purpose of pagination or limiting rows consistently.

Example:

SELECT * FROM employees ORDER BY hire_date OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY;

7. What does FETCH ALL ROWS ONLY do?

The FETCH ALL ROWS ONLY syntax retrieves all rows from the result set, similar to not using the FETCH clause at all. It can be used if you need to ensure that all rows are returned, regardless of any limit.

Example:

SELECT * FROM employees ORDER BY hire_date FETCH ALL ROWS ONLY;

8. How can I implement pagination using OFFSET and FETCH?

Pagination is easily implemented by adjusting the OFFSET value based on the current page. For example:

• Page 1: OFFSET 0 ROWS FETCH NEXT 10 ROWS ONLY
• Page 2: OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY
• Page 3: OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY

This allows users to view different "pages" of results, each containing 10 rows.

9. Does FETCH support LIMIT functionality like MySQL?

Yes, the FETCH clause in Oracle works similarly to the LIMIT clause in MySQL for restricting the number of rows returned. While MySQL uses LIMIT, Oracle uses FETCH (introduced in Oracle 12c) to accomplish the same result.

10. Can FETCH and OFFSET improve performance?

Using FETCH and OFFSET can simplify pagination, but they do not inherently improve performance. In fact, the query still needs to process the entire result set before applying the limit and offset, which may be inefficient for very large datasets.

Performance tips:

• Ensure that you have appropriate indexes on the columns used in the ORDER BY clause.
• Use filters in the WHERE clause to reduce the number of rows as early as possible in the query.

11. What happens if I don't specify FETCH with an OFFSET?

If you use OFFSET without FETCH, Oracle will skip the specified number of rows, but no rows will actually be returned unless you apply another clause (like FETCH NEXT) to retrieve the rows.

Example:

OFFSET 10 ROWS;  -- Skips the first 10 rows but returns no rows

12. Can I use FETCH with ROWNUM or ROW_NUMBER()?

While you can use ROWNUM or ROW_NUMBER() for limiting rows in Oracle SQL, the FETCH and OFFSET clauses are simpler and often more efficient for pagination. However, ROWNUM and ROW_NUMBER() are still commonly used in older codebases.

Example with ROW_NUMBER():

SELECT * FROM (

    SELECT employee_id, first_name, last_name, ROW_NUMBER() OVER (ORDER BY hire_date) AS row_num

    FROM employees

)

WHERE row_num BETWEEN 11 AND 20;

13. Does the FETCH clause affect the execution plan?

The use of FETCH can impact the execution plan because it changes how Oracle retrieves and limits rows. However, unlike ROWNUM or ROW_NUMBER(), FETCH is generally optimized for performance in modern Oracle versions (12c+). You should always review the execution plan to ensure that the query is running efficiently.

14. Can FETCH be used with UNION or JOIN?

Yes, FETCH can be used in queries with UNION, JOIN, or subqueries. However, it's important to keep in mind the behavior of ORDER BY when combining results from multiple queries. If you want to paginate over combined results, apply ORDER BY and FETCH after the UNION or JOIN clause.

Example:

SELECT employee_id, first_name, last_name

FROM employees

UNION

SELECT manager_id, first_name, last_name

FROM managers

ORDER BY last_name

OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY;

15. What is the maximum number of rows I can fetch using FETCH?

There is no specific limit on the number of rows you can fetch using the FETCH clause, other than the maximum number of rows in the underlying table or result set. However, retrieving large numbers of rows without appropriate indexing or optimization might lead to performance issues.