Oracle 18c

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create table temp_emp (
  department varchar2(100 char),
  empname    varchar2(100 char),
  manager varchar2(100 char)
);
BEGIN
insert into temp_emp values ('Sales','John', 'Kevin');
insert into temp_emp values ('Sales','Adam', 'John');
insert into temp_emp values ('Sales','Tom', 'Adam');
insert into temp_emp values ('Sales','Bruce', 'Tom');
insert into temp_emp values ('Marketing','Tony', 'Kevin');
insert into temp_emp values ('Marketing','Bruce', 'Tony');
insert into temp_emp values ('Marketing','John', 'Tony');
END;
/
1 rows affected

SELECT t.*,
       SYS_CONNECT_BY_PATH(manager,'/')||'/'||empname AS hierarchy
FROM   temp_emp t
START WITH (department, manager) NOT IN (SELECT department, empname
                                         FROM   temp_emp)
CONNECT BY
       PRIOR department = department
AND    PRIOR empname    = manager;


DEPARTMENT
EMPNAME
MANAGER
HIERARCHY


Marketing
Tony
Kevin
/Kevin/Tony

Marketing
Bruce
Tony
/Kevin/Tony/Bruce

Marketing
John
Tony
/Kevin/Tony/John

Sales
John
Kevin
/Kevin/John

Sales
Adam
John
/Kevin/John/Adam

Sales
Tom
Adam
/Kevin/John/Adam/Tom

Sales
Bruce
Tom
/Kevin/John/Adam/Tom/Bruce

DEPARTMENT	EMPNAME	MANAGER	HIERARCHY
Marketing	Tony	Kevin	/Kevin/Tony
Marketing	Bruce	Tony	/Kevin/Tony/Bruce
Marketing	John	Tony	/Kevin/Tony/John
Sales	John	Kevin	/Kevin/John
Sales	Adam	John	/Kevin/John/Adam
Sales	Tom	Adam	/Kevin/John/Adam/Tom
Sales	Bruce	Tom	/Kevin/John/Adam/Tom/Bruce