By Michael Nocito , data analyst · Published August 7, 2026 By the end of this page you can check any table for duplicate rows, list every copy, and mark which one to keep, all with queries you understand. You will also know the step that comes before any query: deciding what "duplicate" means for this table, because two rows can match on everything or on one column, and those are different problems with different fixes. It is about twenty minutes. Here is what to actually do with it. On ...