Learn SQL Database Programming

Understanding what a SELF JOIN is and how to use it

A SELF JOIN is used to join a table to itself. This join would be useful in the case of a table containing hierarchical data such as employees and managers. Here, we have a table named Employees containing the following columns and rows:

EmployeeID	FirstName	LastName	ManagerID
1	Jane	Smith	NULL
2	Peter	Jones	1
3	Jessica	Lewis	2
4	Donna	Nickols	2
5	Joel	Rogers	3
6	Joseph	Edwards	7
7	Ruth	Chapman	3
8	Theodore	Clark	6
9	Adam	Berry	6
10	Lucy	Slater	5

You can use a query such as this to self-join on the preceding table, as follows:

SELECT e.FirstName + ' ' + e.LastName AS EmployeeName, ...