Tech moves fast! Stay ahead of the curve with Techopedia!
Join nearly 200,000 subscribers who receive actionable tech insights from Techopedia.
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.
As an example, there is a table named EMPLOYEES that contains three columns:
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.
Do you work in the tech industry? Help us learn more about why the gender gap still exists in tech by taking this quick survey! Survey respondents will also be entered to win a $100 Amazon Gift Card!