Query delete duplicate rows in SQL code example

Example 1: how to remove duplicate in sql

Distinct: helps to remove all the duplicate
records when retrieving the records from a table.

SELECT DISTINCT FIRST_NAME FROM VISITORS;

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