Identifying and removing duplicates

Relational databases work on the idea that items of data can be uniquely identified. However hard we try, there will always be bad data arriving from somewhere. This recipe shows you how to diagnose that and clean up the mess.

Getting ready

Let's start by looking at our example table, cust. It has a duplicate value in customerid:

postgres=# SELECT * FROM cust;
 customerid | firstname | lastname | age
------------+-----------+----------+-----
          1 | Philip    | Marlowe  |  38
          2 | Richard   | Hannay   |  42
          3 | Holly     | Martins  |  25
          4 | Harry     | Palmer   |  36
          4 | Mark      | Hall     |  47
(5 rows)

Before you delete duplicate data, remember that sometimes it isn't the data that is wrong: it is your understanding of it. In those cases, it may be that you haven't properly normalized your database model, and that you need to include additional tables to account for the shape of the data. You might also find...