Which statement describes a SQL window function?

• A. ROW_NUMBER() OVER (ORDER BY date) assigns a unique sequential integer to each row.
• B. ROW_NUMBER() OVER is an aggregate that groups rows.
• C. Window functions are used to join tables.
• D. A window function returns the sum of a column for the entire table.

Answer: (A)

Window functions compute a value for each row while keeping all rows in the result, using the set of surrounding rows defined by the OVER clause. In this example, ROW_NUMBER() OVER (ORDER BY date) assigns a unique sequential integer to every row based on the date order. The numbering starts at 1 and increases by 1 for each subsequent row, even if dates tie, because ROW_NUMBER() guarantees uniqueness. This demonstrates how a window function can provide per-row, position-based values without collapsing rows or performing a table-wide aggregate. The other statements don’t capture this behavior: window functions aren’t about joining tables, and this particular form isn’t just summing across the entire table.