1,2,3,4,5,6,7

create view iterator (x)
as
select 1 union all
select 2 union all
select 3 union all
select 4 union all
select 5
;

--
select substring_index(substring_index(p.category,',',iter.x), ',', -1) as category,
       p.title
  from post p,
       iterator iter
 where iter.x <= length(p.category)-length(replace(p.category,',',''))+1
 order by p.title, iter.x
;

MariaDB [test]> 
MariaDB [test]> 
MariaDB [test]> -- Show the data in the "post" table
MariaDB [test]> select * from post;
+----------+---------+
| category | title   |
+----------+---------+
| 7,3      | title 1 |
| 1,3      | title 2 |
| 1,2,3    | title 3 |
+----------+---------+
3 rows in set (0.00 sec)

MariaDB [test]> 
MariaDB [test]> -- Show the contents of my script "test_qry.sql"
MariaDB [test]> \! cat -n test_qry.sql
     1    --
     2    select substring_index(substring_index(p.category,',',iter.x), ',', -1) as category,
     3           p.title
     4      from post p,
     5           iterator iter
     6     where iter.x <= length(p.category)-length(replace(p.category,',',''))+1
     7     order by p.title, iter.x
     8    ;
     9    
MariaDB [test]> 
MariaDB [test]> -- Run the script "test_qry.sql"
MariaDB [test]> source test_qry.sql
+----------+---------+
| category | title   |
+----------+---------+
| 7        | title 1 |
| 3        | title 1 |
| 1        | title 2 |
| 3        | title 2 |
| 1        | title 3 |
| 2        | title 3 |
| 3        | title 3 |
+----------+---------+
7 rows in set (0.00 sec)

MariaDB [test]> 
MariaDB [test]> 
MariaDB [test]>