mysql的sql语句怎么写

Question

select FolderId,count(*) count from cp_r_morefolder_url group by FolderId 
通过这条语句，我能找出与每个Folder对应的总记录条数，下面我想把这个count设到另一张表中的一个字段中，
另一张表也有FolderId,通过这个字段与之对应，请教各位高手这个sql语句应该怎么写？
update cp_b_morefolder_url set CountApp = () 

Answer 1

update  cp_b_morefolder_url A,
(select FolderId,count(*) co from cp_r_morefolder_url group by FolderId )B
set A.CountApp=B.co
where A.FolderId = B.FolderId

Answer 2

update cp_b_morefolder_url a join (
 select FolderId,count(*) count from cp_r_morefolder_url group by FolderId  
) b on a.FolderId=b.FolderId 
set a.appcount=b.Count

Answer 3

update cp_b_morefolder_url 
inner join ( select FolderId,count(*) as CountApp from cp_r_morefolder_url group by FolderId ) t
on cp_b_morefolder_url.FolderId=t.FolderId
set cp_b_morefolder_url.CountApp=t.CountApp