Use `GROUP BY` with `HAVING COUNT(*) > 1`
Select the duplicate key columns and count the occurrences
Use `ROW_NUMBER() OVER (PARTITION BY … ORDER BY …)` to mark duplicates
Filter rows where `ROW_NUMBER() > 1`
Use `RANK()` or `DENSE_RANK()` when needed for grouped duplicate detection
Join the table to a subquery that returns duplicate keys
Use `EXISTS` with a grouped subquery to find matching duplicate rows
Use `DISTINCT` only to inspect unique values, not to identify duplicates
Include all columns that define a duplicate record in the `PARTITION BY` or `GROUP BY`
Use `COUNT(*) OVER (PARTITION BY …)` to show duplicate counts per row