Oracle Database Interview Question:

How To Write a Query with a Left Outer Join in Oracle?

Submitted by: Administrator

If you want to query from two tables with a left outer join, you can use the LEFT OUTER JOIN ... ON clause in the FROM clause. The following query returns output with a left outer join from two tables: departments and employees. The join condition is that the manager ID in the departments table equals to the employee ID in the employees table:

SQL> set NULL 'NULL'
SQL> SELECT d.department_name, e.first_name, e.last_name
2 FROM departments d LEFT OUTER JOIN employees e
3 ON d.manager_id = e.employee_id;
<pre>DEPARTMENT_NAME FIRST_NAME LAST_NAME
-------------------- -------------------- --------------
Administration Jennifer Whalen
Marketing Michael Hartstein
Purchasing Den Raphaely
Human Resources Susan Mavris
Shipping Adam Fripp
IT Alexander Hunold
......
Treasury NULL NULL
Corporate Tax NULL NULL
Control And Credit NULL NULL
Shareholder Services NULL NULL
Benefits NULL NULL
Manufacturing NULL NULL
Construction NULL NULL

Submitted by: Administrator

Note that a left outer join may return extra rows from the first (left) table that do not satisfy the join condition. In those extra rows, columns from the second (right) table will be given null values.

The extra rows returned from the left outer join in this example represents departments that have no manager IDs.
Submitted by: Administrator