select CASE WHEN u.pay_method = '' THEN 'error' ELSE u.pay_method END AS pay_method, 
count(*) as cnt
from user_client_log u
join product_info p
on u.product_id = p.product_id
where p.product_name = 'anta'
and step = 'select'
group by u.pay_method
order by cnt desc

...