ORA-01427 UPDATE自加入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;