How to get the column name of the result of a least function?

I have an employee table that looks like this:

| id | name | q1 | q2 | q3 | q4 |
+----+------+----+----+----+----+
| 1  | John | 20 | 30 | 10 | 4  |
| 2  | Ram  | 07 | 10 | 03 | 4  |
| 3  | John | 05 | 03 | 15 | 40 |
| 4  | Sree | 12 | 05 | 20 | 25 |

I needed to get the minimum value and maximum value of questions where the id is equal to 4. In this case, I needed 5 and 25 returned. I acheived that using the following query:

SELECT id, name,
  LEAST(q1, q2, q3, q4) AS minValue,
  GREATEST(q1, q2, q3, q4) AS maxValue
FROM employee
WHERE id = 4;

But what this doesn’t return is the question id. How can I adjust my query to show that q2 is the minimum and q4 is the maximum? I know I could write a big case statement, but I also feel like it could be accomplished using a join but I can’t figure it out.

Note: This is for a postgresql database, but I tagged MySQL as well because I know it also supports the LEAST and GREATEST functions. If the solution is very different for both, then I will remove this note and make a separate question.

EDIT

I have an SQL Fiddle already.

Answer

You can use a case statement:

CASE
WHEN LEAST(q1, q2, q3, q4) = q1 THEN 'q1'
WHEN LEAST(q1, q2, q3, q4) = q2 THEN 'q2'
WHEN LEAST(q1, q2, q3, q4) = q3 THEN 'q3'
ELSE 'q4'
END as minQuestion

(Note: it will lose information over ties.)

If you’re interested in ties, approaching it with a subquery and arrays will do the trick:

with employee as (
  select id, q1, q2, q3, q4
  from (values
    (1, 1, 1, 3, 4),
    (2, 4, 3, 1, 1)
  ) as rows (id, q1, q2, q3, q4)
)

SELECT least(q1, q2, q3, q4),
       array(
         select q
         from (values (q1, 'q1'),
                      (q2, 'q2'),
                      (q3, 'q3'),
                      (q4, 'q4')
              ) as rows (v, q)
         where v = least(q1, q2, q3, q4)
       ) as minQuestions
FROM employee e
WHERE e.id = 1;