update statement using self join condition

Question

I have a table call EMPLOYEE, which contains supervisor_id and employee_id. I am trying to update one of the employee(114) supervisor (100), to become the supervisor for 114 employees.

Employee_id	Supervisor_id
114	100
115	114
116	114
117	114
118	114
119	114

I tried using this update statement:

UPDATE EMPLOYEE AS E1
JOIN (SELECT employee_id 
FROM EMPLOYEE
     WHERE supervisor_id = 114) AS E2
     SET E1.supervisor_id = E2.employee_id
     WHERE E1.supervisor_id = 114;

But it printed 115 as the supervisor_id, when it was supposed to print 110. Is there anything wrong with the statement?
Note: I can only use update statement only.

Answer 1

What I feel is that your join operation is giving some different output, can you try to join on table entry like below:

UPDATE EMPLOYEE AS E1
JOIN (SELECT *
FROM EMPLOYEE 
     WHERE supervisor_id = 114) AS E2
     SET E1.supervisor_id = E2.employee_id
     WHERE E1.supervisor_id = 114;

Answer 2

A suggestion, before executing an update with inner join try the same query using select statement to view the rows returned. In your case you can't use update with join because the condition supervisor_id = 114 will remove the value of supervisor_id=100 .

You should use subquery ,but in update or delete you cant specify the same table , so you should wrap the subquery into an outer query giving the table an alias.

This nested query is not very good for performance.

Try:

update employee set supervisor_id= ( select t.supervisor_id 
                                     from ( SELECT supervisor_id 
                                            FROM   employee 
                                            WHERE  employee_id = 114
                                             ) as t 
                                    )
where  supervisor_id=114; 

Demo: https://www.db-fiddle.com/f/qff694udysNgqbyJyFcDzn/8

Result: