MySQL - concatenar valores nul mysql

Tengo un problema, no puedo concatenar todos los campos en una columna ya que algunos campos estan nul y no me salen los registros que tienen un campo nul. Dejo las consultas y la información por si alguien me puede ayudar.

mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+--------+-----------+------+------------+------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600 | 300 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250 | 500 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300 | NULL | 10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)

mysql> select concat(empno,',',ename,',',job,',',mgr,',',hiredate,',',sal,',',co
mm,',',deptno) as todos from emp;
+---------------------------------------------------+
| todos |
+---------------------------------------------------+
| NULL |
| 7499,ALLEN,SALESMAN,7698,1981-02-20,1600,300,30 |
| 7521,WARD,SALESMAN,7698,1981-02-22,1250,500,30 |
| NULL |
| 7654,MARTIN,SALESMAN,7698,1981-09-28,1250,1400,30 |
| NULL |
| NULL |
| NULL |
| NULL |
| 7844,TURNER,SALESMAN,7698,1981-09-08,1500,0,30 |
| NULL |
| NULL |
| NULL |
| NULL |
+---------------------------------------------------+
14 rows in set (0.00 sec)

puedes tulizar lo sguiente..

isnull(COMM ,'',comm)