SQL join manager from same table onto a row with their employees - sql

Question

To start, here's a dummy table I've made to show the data I'm working with:
employee
title
division
email
Boss Person
boss
o
bp#email
John Smith
supervisor
a
jos#email
Jane Smith
supervisor
b
jas#email
Leo Messi
employee
a
lm#email
Amanda Kessel
employee
a
ak#email
Derek Jeter
employee
b
dj#email
I want to end up with the following info:
employee
title
division
email
supervisor_name
supervisor_email
Boss Person
boss
o
bp#email
NULL
NULL
John Smith
supervisor
a
jos#email
Boss Person
bp#email
Jane Smith
supervisor
b
jas#email
Boss Person
bp#email
Leo Messi
employee
a
lm#email
John Smith
jos#email
Amanda Kessel
employee
a
ak#email
John Smith
jos#email
Derek Jeter
employee
b
dj#email
Jane Smith
jas#email
I've looked through and tried documentation at:
https://www.sqltutorial.org/sql-self-join/
SQL Server : LEFT JOIN EMPLOYEE MANAGER relationship
One of the big differences here is I don't have any employee or manager id column to work with.
If you're a supervisor for a division, ie John Smith is a supervisor in division a, then you manage all the employees in division a. Meanwhile, all the supervisors answer to the boss in division o, while the boss answers to no one.
Here is the best code I've tried so far:
select e.*, b.employee as supervisor, b.email as supervisor_email
from employees e, employees b
where b.division = e.division
and
b.title like '%supervisor%'
This got me close, it returned:
employee
title
division
email
supervisor_name
supervisor_email
John Smith
supervisor
a
jos#email
John Smith
jos#email
Jane Smith
supervisor
b
jas#email
Jane Smith
jas#email
Leo Messi
employee
a
lm#email
John Smith
jos#email
Amanda Kessel
employee
a
ak#email
John Smith
jos#email
Derek Jeter
employee
b
dj#email
Jane Smith
jas#email
So, it got the employee info right, but left out the Boss record and placed the supervisors as their own supervisor. I think I need some kind of case or if statement here, but I'm not sure.
Please let me know if this makes sense or if any further clarification is needed.

Answer 1

You could try using a LEFT JOIN and work with two conditions:
when division is the same and we're dealing with the relationship employee < supervisor
when the relationship is supervisor < boss
Here's how I did it:
SELECT t1.*,
t2.employee,
t2.email
FROM tab t1
LEFT JOIN tab t2
ON (t1.division = t2.division AND
t2.title = 'supervisor' AND
t1.title = 'employee')
OR (t2.title = 'boss' AND
t1.title = 'supervisor')
You'll find an SQL fiddle here.

Answer 2

If you want to update the current table (if columns are available), you can do the following (more or less the same as #lemon) :
UPDATE testing t1 JOIN testing t2 ON t2.`division`=t1.division OR t2.division="o" SET
t1.supervisor_name=t2.`employee`, t1.supervisor_email=t2.email
WHERE (CASE
WHEN t1.`title`="employee" THEN t2.title="supervisor"
WHEN t1.`title`="supervisor" THEN t2.title="boss"
END);
SELECT * FROM testing;