SQL: combine two tables with JOIN

"The notebook said: three soaps to Mrs Essomba. The screen tells me: product 5, customer 1. I have a shop to run, not numbers to decode." This is SQL's wall, and it gets a workshop of its own. No functions, no decorative aliases: linking two tables, and understanding what the operation keeps and what it throws away. You will start by answering yourself the question workshop 1 left open: why the till records a number rather than a name. The answer fits in one sentence that holds every database in the world together: a piece of information is written in one place only. Then you will see 218 sales disappear at a stroke. A strict join on customers takes the shop from 320 sales down to 102, without the slightest error message, because two thirds of people pay cash without leaving their name. That is the difference between JOIN and LEFT JOIN, and it is never chosen out of habit: it is chosen by the question you are asking. A whole chapter is devoted to the reflex that will one day stop you announcing a wrong figure: counting the rows after each join and comparing with the starting total. A join does not only lose rows, it can also invent some, and no message will warn you. Finally you will learn the anti-join, the pattern that answers every "who has never" question: customers with no order, invoices with no payment, products nobody ever buys on credit. The workshop ends with a challenge, Mama Ngo's five biggest debtors, whose answer is folded under a ribbon you only pull after searching.