Finding the next available value in MySQL - mysql

Question

I have a database table products with the following columns.
ID | segment_key | segment_value
1 | Mo | 1
2 | Mo | 3
4 | Jo | 1
5 | Jo | 2
6 | Ta | 1
For any given key I need to find the next available segment_value for me to record in the same table.
ie. for the following segment_key list, the expected outputs are
Mo -> 2
Jo -> 3
Ta -> 2
Ji -> 1
I tried the solution mentioned here but I cannot seem to get the right output.
This is my failed attempt.
SELECT t1.segment_value
FROM products t1
WHERE NOT EXISTS (
SELECT *
FROM products t2
WHERE t2.segment_value = t1.segment_value + 1 and t2.segment_key='Mo' and t2.is_active=1
)
LIMIT 1

Answer 1

You can try to use CTE RECURSIVE to get the gap of all values. then do CROSS JOIN fill in the gap of value from each segment_key.
Final using OUTER JOIN and filter segment_key IS NULL which represent the gap of values
Query #1
WITH RECURSIVE CTE AS(
SELECT MIN(segment_value) val,MAX(segment_value) + 1 max_val
FROM products
UNION ALL
SELECT val + 1 ,max_val
FROM CTE c
WHERE val + 1 <= max_val
)
SELECT c.segment_key,MIN(val) segment_value
FROM (
SELECT DISTINCT val,segment_key
FROM CTE
CROSS JOIN products
) c
LEFT JOIN products p
ON c.val = p.segment_value AND c.segment_key = p.segment_key
WHERE p.segment_key IS NULL
GROUP BY c.segment_key;
segment_key
segment_value
Mo
2
Jo
3
Ta
2
View on DB Fiddle