Using a window function, how would you find duplicate rows in a table using SQL?

You can use the ROW_NUMBER() window function to assign a unique sequence number to each row within a partition defined by the columns that should be unique. Rows with a row number greater than 1 are duplicates. Example:

SELECT *
FROM (
  SELECT t.*, ROW_NUMBER() OVER (PARTITION BY col1, col2, col3 ORDER BY id) AS rn
  FROM my_table t
) dup
WHERE rn > 1;

This query partitions the table by the columns that define uniqueness (col1, col2, col3). The ORDER BY clause can be any deterministic column, often the primary key. Complexity is O(n log n) due to sorting within each partition, but most RDBMS implement window functions efficiently. The result returns all duplicate rows; you can then decide to delete or update them as needed.