ALERT

[TIME RUNNING OUT] Discovery & Management Across the Database Environment

Self-Join

Definition - What does Self-Join mean?

A self-join, also known as an inner join, is a structured query language (SQL) statement where a queried table is joined to itself. The self-join statement is necessary when two sets of data, within the same table, are compared.

Techopedia explains Self-Join

As an example, there is a table named EMPLOYEES that contains three columns:

  • employee name
  • employee ID
  • employee manager’s ID

Because the managers are also employees, the MANAGER_ID column also contains the ID of another employee that is also the manager. To write a query to extract the employee and manager names and IDs, the table must be logically split in half to run two separate queries: employees (first table) and managers (second table). This is achieved by running the following sample SQL query:

SELECT a.employee_name, b.employee_name as Manager_name
FROM employees as a, employees as b
WHERE a.manager_id = b.employee_id

Understanding the self-join concept and circumstances is essential to grasping the above SQL statement.

In the example, the second EMPLOYEES table is given the alias b, which actually is a subset of the full EMPLOYEES table. However, the WHERE condition forces the first EMPLOYEES table to query the employee manager in the second EMPLOYEES table.

Share this:

Techopedia Deals

Connect with us

Techopedia on Linkedin
Techopedia on Linkedin
Tweat cdn.techopedia.com
"Techopedia" on Twitter


'@Techopedia'
Sign up for Techopedia's Free Newsletter!

Email Newsletter

Join thousands of others with our weekly newsletter

Resources
The 4th Era of IT Infrastructure: Superconverged Systems
The 4th Era of IT Infrastructure: Superconverged Systems:
Learn the benefits and limitations of the 3 generations of IT infrastructure – siloed, converged and hyperconverged – and discover how the 4th...
Approaches and Benefits of Network Virtualization
Approaches and Benefits of Network Virtualization:
Businesses today aspire to achieve a software-defined datacenter (SDDC) to enhance business agility and reduce operational complexity. However, the...
Free E-Book: Public Cloud Guide
Free E-Book: Public Cloud Guide:
This white paper is for leaders of Operations, Engineering, or Infrastructure teams who are creating or executing an IT roadmap.
Free Tool: Virtual Health Monitor
Free Tool: Virtual Health Monitor:
Virtual Health Monitor is a free virtualization monitoring and reporting tool for VMware, Hyper-V, RHEV, and XenServer environments.
Free 30 Day Trial – Turbonomic
Free 30 Day Trial – Turbonomic:
Turbonomic delivers an autonomic platform where virtual and cloud environments self-manage in real-time to assure application performance.