select * from dept; select * from emp;
+--------+----------+
| deptID | name     |
+--------+----------+
|      1 | pers     |
|      2 | sales    |
|      3 | research |
+--------+----------+

+-------+-------+--------+
| empID | name  | deptID |
+-------+-------+--------+
|     1 | John  |      1 |
|     2 | Fred  |      2 |
|     3 | Susan |      2 |
+-------+-------+--------+

select distinct dept.deptID, emp.deptID 
    from dept left join emp using(deptID);
+--------+--------+
| deptID | deptID |
+--------+--------+
|      1 |      1 |
|      2 |      2 |
|      3 |   NULL |
+--------+--------+