There is an inner and an outer tree beneath every join node, which function similarly to an inner and outer program loop. By convention, the left-hand tree is called the outer tree, and the right-hand tree is called the inner tree.

There are various types of join nodes, described individually below, but the joining method is the same: for each row from the outer tree, there is a join to all of the rows that can possibly qualify from the inner tree. The next row from the outer tree is processed, and so on.

Any join node can have outer join information if an outer join is present.