remove data redundancy in sql code example

Example 1: sql server delete records that have a single duplicate column

WITH cte AS (
    SELECT 
        contact_id, 
        first_name, 
        last_name, 
        email, 
        ROW_NUMBER() OVER (
            PARTITION BY 
                first_name, 
                last_name, 
                email
            ORDER BY 
                first_name, 
                last_name, 
                email
        ) row_num
     FROM 
        sales.contacts
)
DELETE FROM cte
WHERE row_num > 1;

Example 2: query delete duplicates

DELETE FROM dups a USING (
      SELECT MIN(ctid) as ctid, key
        FROM dups 
        GROUP BY key HAVING COUNT(*) > 1
      ) b
      WHERE a.key = b.key 
      AND a.ctid <> b.ctid