Aggregates and Grouping

Part of the PostgreSQL course

Somebody asks you how the shop did last month. They do not want the four thousand order rows. They want one number, or maybe one number per customer, and if you pull the rows out and add them up in application code you have just written a slow, fragile version of something the database does natively and instantly. An aggregate is a function that takes many rows and returns one value: a count, a sum, an average.

Open the terminal, give this lesson a fresh database of its own, and connect to it as postgres (lesson 02 explains the sudo -u postgres) ...