Using DISTINCT for specific columns

Question

select distinct  employee_id, first_name,  commission_pct, department_id from
employees;

When I use the above query it results in distinct combination

Answer 1

You have to remove two columns before distinct

select distinct commission_pct, department_id from
employees; 

Indeed, if your second query would work, what do you expect to see in the first two columns? Consider example data

| employee_id | first_name | commission_pct | department_id |
|     1       |     "x"    |      "b"       |       3       |
|     2       |     "y"    |      "b"       |       3       |
|     1       |     "x"    |      "c"       |       4       |
|     2       |     "y"    |      "c"       |       4       |

You expect to get only two row result like this

| employee_id | first_name | commission_pct | department_id |
|     ?       |      ?     |      "b"       |       3       |
|     ?       |      ?     |      "c"       |       4       |

But what do you expect in the first two column?