How Can Self Joins Help Relate Data Within a Single Table?

In-depth understanding of self-connection: a guide

In relational database management systems, self-join is a powerful operation that allows tables to be joined to themselves. This is particularly useful when you need to compare or correlate data from different rows within a table.

When to use self-connection

When a table contains a column that refers to data in the same table, you need to use a self-join. A common example is an employees table, which contains a SupervisorID column that points to the current employee's supervisor.

To perform a self-join, the table is actually split into two instances, often called e1 and e2 respectively. e1 represents the original table, while e2 is its copy. Then, the join condition becomes e1.column = e2.column.

Example

Consider the following employee table:

EmployeeID	FirstName	LastName	SupervisorID
1	John	Doe	2
2	Mary	Smith	NULL
3	Jane	Brown	1

To query the data and retrieve information about employees and their supervisors in a single row, you can use a self-join:

SELECT e1.EmployeeID, 
       e1.FirstName, 
       e1.LastName,
       e1.SupervisorID, 
       e2.FirstName AS SupervisorFirstName, 
       e2.LastName AS SupervisorLastName
FROM Employee e1
LEFT OUTER JOIN Employee e2 ON e1.SupervisorID = e2.EmployeeID

This query will output the following results:

EmployeeID	FirstName	LastName	SupervisorID	SupervisorFirstName	SupervisorLastName
1	John	Doe	2	Mary	Smith
3	Jane	Brown	1	John	Doe

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