How to get the employees with their managers

Question

This is what I want the output to look like:

Employee   Emp# Manager Mgr# 
   BLAKE   7698 KING    7839 
   CLARK   7782 KING    7839 
   JONES   7566 KING           


        

Answer 1

This is a classic self-join, try the following:

SELECT e.ename, e.empno, m.ename as manager, e.mgr
FROM
    emp e, emp m
WHERE e.mgr = m.empno

And if you want to include the president which has no manager then instead of an inner join use an outer join in Oracle syntax:

SELECT e.ename, e.empno, m.ename as manager, e.mgr
FROM
    emp e, emp m
WHERE e.mgr = m.empno(+)

Or in ANSI SQL syntax:

SELECT e.ename, e.empno, m.ename as manager, e.mgr
FROM
    emp e
    LEFT OUTER JOIN emp m
        ON e.mgr = m.empno

Answer 2

(SELECT ename FROM EMP WHERE empno = mgr)

There are no records in EMP that meet this criteria.

You need to self-join to get this relation.

SELECT e.ename AS Employee, e.empno, m.ename AS Manager, m.empno
FROM EMP AS e LEFT OUTER JOIN EMP AS m
ON e.mgr =m.empno;

EDIT:

The answer you selected will not list your president because it's an inner join. I'm thinking you'll be back when you discover your output isn't what your (I suspect) homework assignment required. Here's the actual test case:

> select * from emp;

 empno | ename |    job    | deptno | mgr  
-------+-------+-----------+--------+------
  7839 | king  | president |     10 |     
  7698 | blake | manager   |     30 | 7839
(2 rows)

> SELECT e.ename employee, e.empno, m.ename manager, m.empno
FROM emp AS e LEFT OUTER JOIN emp AS m
ON e.mgr =m.empno;

 employee | empno | manager | empno 
----------+-------+---------+-------
 king     |  7839 |         |      
 blake    |  7698 | king    |  7839
(2 rows)

The difference is that an outer join returns all the rows. An inner join will produce the following:

> SELECT e.ename, e.empno, m.ename as manager, e.mgr
FROM emp e, emp m
WHERE e.mgr = m.empno;

 ename | empno | manager | mgr  
-------+-------+---------+------
 blake |  7698 | king    | 7839
(1 row)

Answer 3

Perhaps your subquery (SELECT ename FROM EMP WHERE empno = mgr) thinks, give me the employee records that are their own managers! (i.e., where the empno of a row is the same as the mgr of the same row.)

have you considered perhaps rewriting this to use an inner (self) join? (I'm asking, becuase i'm not even sure if the following will work or not.)

SELECT t1.ename, t1.empno, t2.ename as MANAGER, t1.mgr
from emp as t1
inner join emp t2 ON t1.mgr = t2.empno
order by t1.empno;

Answer 4

TRY THIS

SELECT E.ename,E.empno,ISNULL(E.ename,'NO MANAGER') AS MANAGER FROM emp e
INNER JOIN emp M
ON  M.empno=E.empno

Instaed of subquery use self join

Answer 5

You could have just changed your query to:

SELECT ename, empno, (SELECT ename FROM EMP WHERE empno = e.mgr)AS MANAGER, mgr 
from emp e 
order by empno;

This would tell the engine that for the inner emp table, empno should be matched with mgr column from the outer table.