Self-join

A self-join is joining a table to itself.[6]
[edit]Example
A query to find all pairings of two employees in the same country is desired. If there were two separate tables for employees and a query which requested employees in the first table having the same country as employees in the second table, a normal join operation could be used to find the answer table. However, all the employee information is contained within a single large table.[7]
Consider a modified Employee table such as the following:
Employee Table
EmployeeID	LastName	Country	DepartmentID
123	Rafferty	Australia	31
124	Jones	Australia	33
145	Steinberg	Australia	33
201	Robinson	United States	34
305	Smith	Germany	34
306	John	Germany	NULL

An example solution query could be as follows:
SELECT F.EmployeeID, F.LastName, S.EmployeeID, S.LastName, F.Country
FROM Employee F
INNER JOIN Employee S ON F.Country = S.Country
WHERE F.EmployeeID < S.EmployeeID
ORDER BY F.EmployeeID, S.EmployeeID;
Which results in the following table being generated.
Employee Table after Self-join by Country
EmployeeID	LastName	EmployeeID	LastName	Country
123	Rafferty	124	Jones	Australia
123	Rafferty	145	Steinberg	Australia
124	Jones	145	Steinberg	Australia
305	Smith	306	John	Germany

For this example:
F and S are aliases for the first and second copies of the employee table.
The condition F.Country = S.Country excludes pairings between employees in different countries. The example question only wanted pairs of employees in the same country.
The condition F.EmployeeID < S.EmployeeID excludes pairings where the EmployeeID of the first employee is greater than or equal to the EmployeeID of the second employee. In other words, the effect of this condition is to exclude duplicate pairings and self-pairings. Without it, the following less useful table would be generated (the table below displays only the "Germany" portion of the result):
EmployeeID	LastName	EmployeeID	LastName	Country
305	Smith	305	Smith	Germany
305	Smith	306	John	Germany
306	John	305	Smith	Germany
306	John	306	John	Germany

Only one of the two middle pairings is needed to satisfy the original question, and the topmost and bottommost are of no interest at all in this example.