What is a self join? Explain it with an example.

Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.

CREATE TABLE emp

(

empid int,

mgrid int,

empname char(10)

)

INSERT emp SELECT 1,2,’Vyas’

INSERT emp SELECT 2,3,’Mohan’

INSERT emp SELECT 3,NULL,’Shobha’

INSERT emp SELECT 4,2,’Shridhar’

INSERT emp SELECT 5,2,’Sourabh’

SELECT t1.empname [Employee], t2.empname [Manager]

FROM emp t1, emp t2

WHERE t1.mgrid = t2.empid

Here is an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)

SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager]

FROM emp t1

LEFT OUTER JOIN

emp t2

ON

t1.mgrid = t2.empid