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 an example table, cust. It has a duplicate value in customerid:

CREATE TABLE cust (
 customerid BIGINT NOT NULL
,firstname  TEXT NOT NULL
,lastname   TEXT NOT NULL
,age       INTEGER NOT NULL);
INSERT INTO cust VALUES (1, 'Philip', 'Marlowe', 33);
INSERT INTO cust VALUES (2, 'Richard', 'Hannay', 37);
INSERT INTO cust VALUES (3, 'Harry', 'Palmer', 36);
INSERT INTO cust VALUES (4, 'Rick', 'Deckard', 4);
INSERT INTO cust VALUES (4, 'Roy', 'Batty', 41);
postgres=# SELECT * FROM cust ORDER...