what is wrong with this query?geeting wrong count : Table1: mas_book_author

author_id|    author_name     
------------+-------------
 1 | jhon
 2 | roobini
 3 | virat
 4 | sachin
 5 | siva
 6 | priya

Table4: mas_bk_accession_entry

id(int)(pk)  | author_ids(varchar)
-------------+-------------------
1            | 1,5
2            | 5,1
3            |2,5
4            |3,5
5            |4,5  

my result should be like this:

author_name |count|
----------+--------
 jhon,siva  |2    

This is my Query

select b.author_name,COUNT(*) as totbook from mas_bk_accession_entry a
join mas_book_author as b on b.author_id in (select cast((unnest(string_to_array(f.author_ids,',')) ) as int) as author_ids from mas_bk_accession_entry f)
where a.author_ids = '1,5'
group by b.author_name

