Combining tables with joins

Real questions span tables: orders and customers, events and users. Joins put them back together.

Inner join

Rows that match on both sides:

SELECT c.name, o.amount
FROM orders o
JOIN customers c ON c.id = o.customer_id;

Left join

Keep every row on the left, matched or not — the go-to for "including the ones with zero":

SELECT c.name, count(o.order_id) AS orders
FROM customers c
  orders o  o.customer_id  c.id
  c.name;