Question

Say I have a table ‘orders’ created as:

CREATE TABLE orders (id SERIAL,                       customerID INTEGER,                      timestamp BIGINT,                       PRIMARY KEY(id)); 

Timestamp being the UNIX timestamp. Now i want to select the ids of the LATEST orders for every customer. As a view would be nice.

however the following statement

CREATE VIEW lastOrders AS SELECT  id,                                   customerID,                                    MAX(timestamp)                                    FROM orders                                    GROUP BY customerID; 

Causes a postgre error:

ERROR: column ‘orders.id’ must appear in the GROUP BY clause or be used in an aggregate function

What am I doing wrong?

Answer 1

For these kind of things you can use 2 approaches. One has been already shown by Jens.

Other, is to use ‘DISTINCT ON’ clause:

CREATE VIEW lastOrders AS SELECT DISTINCT ON (customerID)     id,     customerID,     timestamp FROM orders ORDER BY customerID, timestamp desc;