Mastering PostgreSQL 9.6

Creating simple aggregates

For the purpose of this example the goal is to solve a very simple problem: if you take a taxi, you usually have to pay for getting in the taxi (for example, 2.50 EUR). Then let us assume that for each kilometer the customer has to pay 2.20 EUR. The question now is: what is the total price of a trip?

Of course, this example is simple enough to solve the problem without a custom aggregate, however, let's see how it works. First some test data is created:

test=# CREATE TABLE t_taxi (trip_id int, km numeric); CREATE TABLE test=# INSERT INTO t_taxi VALUES     (1, 4.0), (1, 3.2), (1, 4.5),    (2, 1.9), (2, 4.5); INSERT 0 5 

To create aggregates PostgreSQL offers the CREATE AGGREGATE command. The syntax of this command has ...