DELETE FROM table_with_duplicates AS T1 USING table_with_duplicates AS T2
WHERE
T1.column_1 = T2.column_1
AND T1.column_2 = T2.column_2
AND T1.column_3 = T2.column_3
AND T1.row_num < T2.row_num
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Sent from: http://www.postgresql-archive.org/PostgreSQL-general-f1843780.html