สอบถามปัญหาคำสั่ง SQL ของ MySQL เกี่ยวกับการใช้ WHERE ร่วมกับ count

SELECT 
CASE show_main
WHEN 'y' THEN 'y'  ELSE  'n' 
END AS show
FROM prd_pic 

select * from table 
where prod=1111 
and (
   (show_main='Y') 
   or 
   ( show_main='N' and not exists( select * from table where prod=1111 and show_main='Y'))
) 
order by rand() )

select prd_id  ,case when sum(case show_main when 'y' then 1 else 0 end)>0 then 'y' else 'n'  end as show_main 
from prd_pic where  prd_id='1111111' 

select prd_pic.* ,case when tb_y.prd_id is null then 'n' else 'y'  end as show_main2  from prd_pic 
left join (select prd_id  from prd_pic where show_main='y' and prd_id='1111111' limit 1) as tb_y on prd_pic.prd_id=tb_y.prd_id
where prd_pic.prd_id='1111111'