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

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.

2. Why 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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