MySQL Independently select last value across multiple columns which isn't null - mysql

Question

Here's an example dataset that I'm dealing with:
+----+-----+-----+-----+-----+
| id | a | b | c | d |
+----+-----+-----+-----+-----+
| 1 | 1 | | | |
| 2 | | 2 | | |
| 3 | | | | |
| 4 | | | | 4 |
| 5 | | 3 | | |
+----+-----+-----+-----+-----+
I want to select the bottom-most values. If this value has never been set, then I'd want "null", otherwise, I want the bottom-most result. In this case, I'd want the resultset:
+-----+-----+-----+-----+
| a | b | c | d |
+-----+-----+-----+-----+
| 1 | 3 | | 4 |
+-----+-----+-----+-----+
I tried queries such as variations of:
SELECT DISTINCT `a`,`b`,`c`,`d`
FROM `test`
WHERE `a` IS NOT NULL
AND `b` IS NOT NULL
AND `c` IS NOT NULL
AND `d` IS NOT NULL
ORDER BY 'id' DESC LIMIT 1;
This didn't work.
Would I have to run queries for each value individually, or is there a way to do it within just that one query?

Answer 1

If you are OK with changing type to a char, you can do this:
SELECT substring_index(GROUP_CONCAT(a),',',1) as LastA,
substring_index(GROUP_CONCAT(b),',',1) as LastB,
substring_index(GROUP_CONCAT(c),',',1) as LastC,
substring_index(GROUP_CONCAT(d),',',1) as LastD
FROM
(
SELECT id, a, b, c, d
FROM MyTable
ORDER BY id DESC
) x;
SqlFiddle here
Notes:
The intermediate derived table is needed as the input to GROUP_CONCAT needs to be ordered.
After compressing the rows with GROUP_CONCAT (using the default comma delimiter), we then scrape out the first column with substring_index. substring_index on NULL returns NULL, as required.
If you need the resultant columns to be INT, you'll need to cast each column again.