Get column name instead of column value

Question

My table table1 is like below

id   d1   d2   d3   d4
1    6    7    8    9
2    13   10   11   12
3    16   17   14   15
4    19   20   21   18
         


        

Answer 1

In an RDBMS, it would be more usual to store data something like this:

DROP TABLE IF EXISTS my_table;

CREATE TABLE my_table
(id INT NOT NULL
,d INT NOT NULL
,reading INT NOT NULL
,PRIMARY KEY(id,d)
);

INSERT INTO my_table VALUES
(1, 1 ,      6),
(1, 2 ,      7),
(1, 3 ,      8),
(1, 4 ,      9),
(2, 1 ,     13),
(2, 2 ,     10),
(2, 3 ,     11),
(2, 4 ,     12),
(3, 1 ,     16),
(3, 2 ,     17),
(3, 3 ,     14),
(3, 4 ,     15),
(4, 1 ,     19),
(4, 2 ,     20),
(4, 3 ,     21),
(4, 4 ,     18);

SELECT * FROM my_table;
+----+---+---------+
| id | d | reading |
+----+---+---------+
|  1 | 1 |       6 |
|  1 | 2 |       7 |
|  1 | 3 |       8 |
|  1 | 4 |       9 |
|  2 | 1 |      13 |
|  2 | 2 |      10 |
|  2 | 3 |      11 |
|  2 | 4 |      12 |
|  3 | 1 |      16 |
|  3 | 2 |      17 |
|  3 | 3 |      14 |
|  3 | 4 |      15 |
|  4 | 1 |      19 |
|  4 | 2 |      20 |
|  4 | 3 |      21 |
|  4 | 4 |      18 |
+----+---+---------+

The query to extract the desired result could then be something like this...

SELECT a.* 
  FROM my_table a 
  JOIN 
     ( SELECT id,MIN(reading) reading FROM my_table GROUP BY id ) b 
    ON b.id = a.id 
   AND b.reading = a.reading;
+----+---+---------+
| id | d | reading |
+----+---+---------+
|  1 | 1 |       6 |
|  2 | 2 |      10 |
|  3 | 3 |      14 |
|  4 | 4 |      18 |
+----+---+---------+