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

Can you try this one?

SELECT
  NAME1,
  PH
FROM
      (WITH T
           AS (SELECT
                    'mark' NAME1,
                    '1234567' PH
              FROM
                    DUAL
              UNION ALL
              SELECT
                    'bailey',
                    '456789'
              FROM
                    DUAL
              UNION ALL
              SELECT
                    'mark',
                    '987654'
              FROM
                    DUAL)
       SELECT
            NAME1,
            PH,
            ROW_NUMBER ( ) OVER (PARTITION BY NAME1 ORDER BY NAME1) SEQ
       FROM
            T)
WHERE
      SEQ = 1;

If you dont care on a specific row, then use aggregate functions

SELECT
      NAME1,
      MAX ( PH ) PH
FROM
      T
GROUP BY
      NAME1;

Answer 2

What you request is impossible. You cannot select all the employee ids but have only distinct commission_pct and department_id.

So think it over, what you want to show:

• All distinct commission_pct, department_id only?
• All distinct commission_pct, department_id and the number of relevant employees?
• All distinct commission_pct, department_id and the relevant employees comma separated?
• All employees, but with nulls when commission_pct and department_id are the same as in the line before?

The first can be solved with DISTINCT. The second and third with GROUP BY (plus count or listagg). The last would be solved with the analytic function LAG.

Answer 3

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?