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...