ORA-01427 while UPDATE self-join ORACLE - oracle

Question

two tables, each has employee_id and manager_id which links to employee_id Tables have different employee_id.
UPDATE employee u
SET u.manager_id = (SELECT m.id
FROM employee e
JOIN old_db.employees oe ON e.last_name = oe.last_name
JOIN old_db.employees om ON oe.manager_id = om.employee_id
INNER JOIN employee m ON m.last_name = om.last_name
WHERE e.id = u.id)
WHERE manager_id IS NULL;
gives
ERROR at line 2:
ORA-01427: single-row subquery returns more than one row
I've tried rownum=1 but this makes all manager_id same.
Select query with one given employee_id returns one value
SQL> SELECT m.id
2 FROM employee e
3 JOIN old_db.employees oe ON e.last_name = oe.last_name
4 JOIN old_db.employees om ON oe.manager_id = om.employee_id
5 INNER JOIN employee m ON m.last_name = om.last_name
6 WHERE e.id = 1805;
ID
----------
1804

Answer 1

Well the root cause of problem is in table employees or in table employee last_name are not unique. And when you join table on last name you get more then one row. You may check it with next query:
SELECT e.id, count(*), count(distinct m.id)
FROM employee e
JOIN old_db.employees oe ON e.last_name = oe.last_name
JOIN old_db.employees om ON oe.manager_id = om.employee_id
INNER JOIN employee m ON m.last_name = om.last_name
GROUP BY e.id;
May be exists more suitable column to join it for example employee_id if so you query may be rewritten to:
UPDATE employee u
SET u.manager_id = (SELECT oe.manager_id
FROM old_db.employees oe
WHERE oe.employee_id = u.id)
WHERE manager_id IS NULL;