Joins - Joining a table to itself

This is a quick sql query that you can use to join a table to itself. There are a lot of joins, the most common is used to join two related tables together, but the LEFT JOIN can be a really quick alternative to a Sub query.

I used something similar to populate a treeview that had child and parent nodes. Effectively each parent had muliple children, and I used a sql query to check the data that I wanted (I will turn this into a LINQ statement in code, rather than a stored procedure)

use category
select c.ParentID, c.ChildID, c.Description
from category c
left join category p on c.ChildID = p.ParentID and p.ChildID = c.ParentID
order by c.ParentID

More info here: http://www.w3schools.com/sql/sql_join_left.asp