MySQL 8.0

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create table t (post_id int, category_id int, post_title varchar(255))
insert into t
    select 1 as post_id, 1 as category_id, 'Red' as post_title union all
    select 2 as post_id, 1 as category_id, 'Black' as post_title union all
    select 3 as post_id, 2 as category_id, 'Medium' as post_title union all
    select 4 as post_id, 3 as category_id, 'Male' as post_title union all
    select 5 as post_id, 3 as category_id, 'Female' as post_title 
    

Records: 5  Duplicates: 0  Warnings: 0


with recursive tt as (
      select t.*, dense_rank() over (order by category_id) as cat_seqnum
      from t
     ),
     cte as (
      select cat_seqnum, post_title
      from tt
      where cat_seqnum = 1
      union all
      select  tt.cat_seqnum, concat_ws('-', cte.post_title, tt.post_title)
      from cte join
           tt
           on tt.cat_seqnum = cte.cat_seqnum + 1
     )
select *
from cte
where cat_seqnum = (select max(cat_seqnum) from tt)


cat_seqnum
post_title


3
Red-Medium-Male

3
Red-Medium-Female

3
Black-Medium-Male

3
Black-Medium-Female

cat_seqnum	post_title
3	Red-Medium-Male
3	Red-Medium-Female
3	Black-Medium-Male
3	Black-Medium-Female