LEFT JOIN • RIGHT JOIN • FULL JOIN • CROSS JOIN
We now advance from inner joins to outer joins. The main restriction of inner joins is that they require a match in all tables being joined to show any results. If you’re joining a Customers table to an Orders table, no data is shown for the customer if that customer hasn’t yet placed an order. This may seem like a relatively unimportant problem, but it often becomes significant with different types of data.
To use a different example, let’s say we have an Orders table and a Refunds table. The Refunds table is related to the Orders table by OrderID. In other words, all refunds are tied to a specific order. The refund can’t exist unless the order exists. ...