MySQL 8.0

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group_id	a_id	b_id	c_id	d_id
1	1	null	null	null
1	null	2	null	null
1	null	null	3	null
1	null	null	null	4
1	null	null	null	5
2	11	null	null	null
2	null	12	null	null
2	null	null	13	null

WITH 
cte_a AS ( SELECT group_id, a_id, ROW_NUMBER() OVER (PARTITION BY group_id) rn
           FROM test
           WHERE a_id IS NOT NULL ),
cte_b AS ( SELECT group_id, b_id, ROW_NUMBER() OVER (PARTITION BY group_id) rn
           FROM test
           WHERE b_id IS NOT NULL ),
cte_c AS ( SELECT group_id, c_id, ROW_NUMBER() OVER (PARTITION BY group_id) rn
           FROM test
           WHERE c_id IS NOT NULL ),
cte_d AS ( SELECT group_id, d_id, ROW_NUMBER() OVER (PARTITION BY group_id) rn
           FROM test
           WHERE d_id IS NOT NULL ),
cte_n AS ( SELECT group_id, rn FROM cte_a
           UNION
           SELECT group_id, rn FROM cte_b
           UNION
           SELECT group_id, rn FROM cte_c
           UNION
           SELECT group_id, rn FROM cte_d )
SELECT group_id,
       cte_a.a_id,
       cte_b.b_id,
       cte_c.c_id,
       cte_d.d_id
FROM cte_n
LEFT JOIN cte_a USING (group_id, rn)
LEFT JOIN cte_b USING (group_id, rn)
LEFT JOIN cte_c USING (group_id, rn)
LEFT JOIN cte_d USING (group_id, rn)
ORDER BY group_id, rn;

group_id	a_id	b_id	c_id	d_id
1	1	2	3	4
1	null	null	null	5
2	11	12	13	null