[sql] How to get the employees with their managers

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    7839 
   MARTIN  7654 BLAKE   7698 
   ALLEN   7499 BLAKE   7698 
   TURNER  7844 BLAKE   7698 
   JAMES   7900 BLAKE   7698 
   WARD    7521 BLAKE   7698 
   FORD    7902 JONES   7566 
   SMITH   7369 FORD    7902 
   SCOTT   7788 JONES   7566 
   ADAMS   7876 SCOTT   7788 
   MILLER  7934 CLARK   7782 

Here's what I got:

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

 ENAME           EMPNO MANAGER           MGR
 ---------- ---------- ---------- ----------
 SMITH            7369                  7902
 ALLEN            7499                  7698
 WARD             7521                  7698
 JONES            7566                  7839
 MARTIN           7654                  7698
 BLAKE            7698                  7839
 CLARK            7782                  7839
 SCOTT            7788                  7566
 KING             7839
 TURNER           7844                  7698
 ADAMS            7876                  7788

 ENAME           EMPNO MANAGER           MGR
 ---------- ---------- ---------- ----------
 JAMES            7900                  7698
 FORD             7902                  7566
 MILLER           7934                  7782

I can't find why the manager field is blank.

Here's the table:

 SQL> select empno, ename, job,deptno, mgr from emp;

 EMPNO ENAME      JOB           DEPTNO        MGR
 ---------- ---------- --------- ---------- ----------
  7839 KING       PRESIDENT         10
  7698 BLAKE      MANAGER           30       7839
  7782 CLARK      MANAGER           10       7839
  7566 JONES      MANAGER           20       7839
  7654 MARTIN     SALESMAN          30       7698
  7499 ALLEN      SALESMAN          30       7698
  7844 TURNER     SALESMAN          30       7698
  7900 JAMES      CLERK             30       7698
  7521 WARD       SALESMAN          30       7698
  7902 FORD       ANALYST           20       7566
  7369 SMITH      CLERK             20       7902

 EMPNO ENAME      JOB           DEPTNO        MGR
 ---------- ---------- --------- ---------- ----------
  7788 SCOTT      ANALYST           20       7566
  7876 ADAMS      CLERK             20       7788
  7934 MILLER     CLERK             10       7782

14 rows selected.

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.

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

(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)

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;