WHAT IS SELF JOIN AND WHY IS IT REQUIRED? : SQL SELF JOIN EXAMPLE

WHAT IS SELF JOIN AND WHY IS IT REQUIRED?

Self Join is the act of joining one table with itself.

Self Join is often very useful to convert a hierarchical structure into a flat structure

In our employee table example above, we have kept the manager ID of each employee in the same row as that of the employee. This is an example of how a hierarchy (in this case employee-manager hierarchy) is stored in the RDBMS table. Now, suppose if we need to print out the names of the manager of each employee right beside the employee, we can use self join. See the example below:

SELECT e.name EMPLOYEE, m.name MANAGER

FROM EMPLOYEE e, EMPLOYEE m

WHERE e.mgr_id = m.id (+)

EMPLOYEE	MANAGER
Pete	Hash
Darl	Hash
Inno	Hash
Robo	Hash
Tomiti	Robo
Anno	Robo
Privy	Robo
Meme	Pete
Bhuti	Tomiti
Hash

The only reason we have performed a left outer join here (instead of INNER JOIN) is we have one employee in this table without a manager (employee ID = 1). If we perform inner join, this employee will not show-up.