# Find Duplicate Records In Table Without Unique Id

I recently came across a couple methods for listing out instances of duplicate records in a table where the table doesn't have an explicit unique identifier. Here is [a post](../queries/find-records-that-contain-duplicate-values) that explains how to do this when a unique identifier is present.

If the table doesn't have an explicit primary key or other uniquely identifying value, then we'll have to get some help from [PostgreSQL's internal system columns](https://www.postgresql.org/docs/current/ddl-system-columns.html) — namely the `ctid`.

The `ctid` is:

> The physical location of the row version within its table.

Let's use the example of the `mailing_list` table with potential duplicate `email` values.

Here is the [first approach](https://stackoverflow.com/a/26773018/535590):

```sql
delete from mailing_list
where ctid not in (
  select min(ctid)
  from mailing_list
  group by email
);
```

This uses a subquery to find the first occurrence of every unique email and then deletes the rest. The `ctid` is the unique value that we can call the `min` aggregate on.

A [second approach](https://stackoverflow.com/a/46775289/535590):

```sql
delete from mailing_list ml1
  using mailing_list ml2
where ml1.ctid < ml2.ctid
  and ml1.email = ml2.email;
```

This uses `delete using` to join the table against itself as a cartesian product to compare every entry to every other entry.
