I try to find a wordaround to fix this problem.

SELECT count(*)
FROM blog_comment
where comment_parent in ( select log_ID
from blog_content
where log_userID = $ID and deleteFlag = 'N')
order by comment_postTime desc

for subquery, I can rewrite it:

select distinct com.*
from blog_comment com, blog_content con
where com.comment_parent = con.log_ID
and con.log_userID = 7 and con.deleteFlag='N'
ORDER BY com.comment_postTime DESC

but if I need to use count, count (distinct com.*) I got this error message
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '*)
I dont know how to fix it.

If you know a better way to rewrite this sql statement, please let me know