目标

原数据

代码

with cte as (
    select 'A+B' as A,
           1     as b
    UNION ALL
    SELECT 'C+D' as A,
           2   as b
)  
SELECT A,
       substring_index(substring_index(f.A, '+', (h.help_topic_id + 1)), '+', -(1)) as AA,
       b
FROM cte f
         join mysql.help_topic h
              on h.help_topic_id < ((length(f.A) - length(replace(f.A, '+', ''))) + 1)

注：将代码中 ”+“ 修改为连接符号即可

结果