How to Efficiently Limit and Offset Query Results in Oracle?

Mastering Row Limiting and Offsetting in Oracle Queries

Oracle's ROWNUM pseudocolumn, frequently used for result set limitation, presents a unique challenge when combined with ordering. Unlike MySQL's LIMIT clause, which applies after sorting, ROWNUM filters before the ORDER BY clause is processed. This means simply using ROWNUM won't guarantee a specific ordered subset of your results.

The Standard Approach (All Oracle Versions):

To achieve the equivalent of MySQL's LIMIT functionality, use a nested query:

SELECT *
FROM (
    SELECT *
    FROM emp
    ORDER BY sal DESC
)
WHERE ROWNUM <= 5;

This approach first orders the data and then applies the ROWNUM filter, ensuring the top 5 highest salaries are returned.

Offsetting and Limiting Results:

For more sophisticated control, specifying both an offset and limit requires a slightly more complex query:

SELECT *
FROM (
    SELECT a.*, ROWNUM rnum
    FROM (
        SELECT *  -- Your main query with ORDER BY clause
        FROM some_table
        ORDER BY some_column
    ) a
    WHERE ROWNUM <= :MAX_ROW_TO_FETCH
)
WHERE rnum >= :MIN_ROW_TO_FETCH;

Replace :MAX_ROW_TO_FETCH and :MIN_ROW_TO_FETCH with your desired upper and lower bounds. This fetches rows within a specific range after ordering.

Oracle 12c and Beyond: The FETCH Clause:

Oracle 12c (and later versions) offer a more elegant solution using the FETCH clause:

-- Retrieve the first 10 rows
SELECT *
FROM sometable
ORDER BY name
FETCH FIRST 10 ROWS ONLY;

-- Retrieve rows 20-30
SELECT *
FROM sometable
ORDER BY name
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

The FETCH clause provides a cleaner and more readable way to limit and offset results, making it the preferred method for Oracle 12c and later. For older versions, the nested query approach remains necessary.

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