The ROW_NUMBER() function is a window function that assigns a unique sequential integer to each row within a partition of a result set, ordered by a specified column. Think of it as numbering rows in a specific order. This is particularly useful for generating sequential numbers for rows, especially when dealing with ranking or pagination. It's different from aggregate functions like COUNT or SUM, which operate on groups of rows. ROW_NUMBER() operates on individual rows within a partition. Crucially, it assigns a unique number to each row, even if there are ties in the ordering column. This ensures that no two rows have the same number within a partition. This function is often used in conjunction with other SQL clauses like ORDER BY and WHERE to filter and manipulate the results. For example, you might use it to number customer orders in chronological order or to select the top 10 performers from a list of employees.