Using Self-Joins to Access Related Records in the Same Table

You can use a self-join to simplify nested SQL queries where the inner and outer queries reference the same table. These joins allow you to retrieve related records from the same table. The most common case where you'd use a self-join is when you have a table that references itself, such as the employees table shown below:

id first_name last_name manager
----------- --------------- --------------- -----------
1 Pat Crystal NULL
2 Dennis Miller 1
3 Jacob Smith 1

Suppose you're tasked with writing a SQL query to retrieve a list of employees and their managers. You can't write a basic SQL SELECT statement to retrieve this information, as you need to cross reference information contained in other records within the same table. Fortunately, you can use a self-join to solve this dilemma by joining the table to itself.

You can find the whole article at http://databases.about.com/od/sql/a/selfjoins.htm