How Does Oracle's ROWNUM Work for Efficient Database Pagination?

Mastering Oracle's ROWNUM for Database Pagination

Oracle's ROWNUM pseudocolumn is a key tool for retrieving specific row sets, essential for database pagination. This guide addresses common questions and best practices:

*1. Why `SELECT FROM Person WHERE rownum > 100 AND rownum < 200` Fails**

ROWNUM assignment happens after the WHERE clause is processed. Because rownum > 100 is always false for the first row, ROWNUM never increments past 1, resulting in zero rows returned.</p> <p><strong>2. Why <code>SELECT ... FROM ... WHERE rownum BETWEEN lowerBound AND upperBound (Pre-12c)

Before Oracle 12c, direct use of BETWEEN with ROWNUM wasn't possible. Oracle 12c introduced Top-n Row Limiting, offering a more efficient solution.

3. Hiding the ROWNUM Column

To exclude ROWNUM from your results, explicitly list the desired columns in the SELECT statement of your outer query. In SQL*Plus, the COLUMN command provides an alternative for hiding columns.

4. ROWNUM's Accuracy in Pagination

ROWNUM provides reliable pagination when used correctly. Crucially, order your results using a unique or predictable column to maintain consistent pagination across queries.

Key Considerations:

• ROWNUM can impact performance; use judiciously.
• Always order your results for predictable pagination.
• For improved performance in Oracle 12c and later, utilize the OFFSET and FETCH clauses.

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