Nate Johnson continues his nested set series:

Now the complex. To validate that our tree is properly structured, the following statements need to be true:

Each node’s Right value is greater than its Left.

More to the point, each node’s Right value is

**greater**than all of its**ancestors’**Left values.Similarly, each node’s Left value is

**less**than all of its**descendants’**Left values (and Right values, obviously!)Leaf nodes have no gaps between Left & Right:

`Right = Left + 1`

`Depth`

is easy to verify because we already wrote the rCTE to calculate it!And of course,

**no orphans**– all ParentIDs lead to an actual parent node, except of course if they’re`NULL`

(root nodes).

Read on for further explanation of these points.