Self Join

A self-join is simply a normal SQL join that joins one table to itself. This is accomplished by using table name aliases to give each instance of the table a separate name. Joining a table to itself can be useful when you want to compare values in a column to other values in the same column. A join in which records from a table are combined with other records from the same table when there are matching values in the joined fields.

A self-join can be an inner join or an outer join. A table is joined to itself based upon a field or combination of fields that have duplicate data in different records. The data-type of the inter-related columns must be of the same type or needs to cast them in same type.

When all of the data you require is contained within a single table, but data needed to extract is related to each other in the table itself. Examples of this type of data relate to Employee information, where the table may have both an Employee’s ID number for each record and also a field that displays the ID number of an Employee’s supervisor or manager. To retrieve the data tables are required to relate/join to itself.

Another example which can be tried on SQL SERVER 2005 sample database AdventureWorks is to find products that are supplied by more than one vendor.

Please refer the sample database for table structure.

SELECT DISTINCT pv1.ProductID, pv1.VendorID

FROM Purchasing.ProductVendor pv1

INNER JOIN Purchasing.ProductVendor pv2

ON pv1.ProductID = pv2.ProductID

AND pv1.VendorID = pv2.VendorID

ORDER BY pv1.ProductID

Example:

Self Join in SQL Server 2000/2005 helps in retrieving the records having some relation or similarity with other records in the same database table. A common example of employees table can do more clearly about the self join in sql. Self join in sql means joining the single table to itself. It creates the partial view of the single table and retrieves the related records. You can use aliases for the same table to set a self join between the single table and retrieve the records satisfying the condition in where clause.

For self join in sql you can try the following example:

Create table employees:

emp_id emp_name emp_manager_id
1 John Null
2 Tom 1
3 Smith 1
4 Albert 2
5 David 2
6 Murphy 5
7 Petra 5

Now to get the names of managers from the above single table you can use sub queries or simply the self join.

Self Join SQL Query to get the names of manager and employees:

select e1.emp_name 'manager',e2.emp_name 'employee'

from employees e1

join employees e2

on e1.emp_id=e2.emp_manager_id

manager employee
John Tom
John Smith
Tom Albert
Tom David
David Murphy
David Petra

Understanding the Self Join Example

In the above self join query, employees table is joined with itself using table aliases e1 and e2. This creates the two views of a single table.

from employees e1

join employees e2

on e1.emp_id=e2.emp_manager_id

Here e.emp_manager_id passes the manager id from the 2nd view to the first aliased e1 table to get the names of managers.