"count" query

i need to query the following tables to return a category name,category description, username of user who made category and no. of posts in each category.i am using this for a blog i am developing which needs to display a list of categories including category description, user who posted the category and no. of posts in the category
i have been using the following query but doesnt seem to work for me

select C.CATEGORY_NAME,C.DESCRIPTION,U.USER_ID,count(P.PO ST_ID)
from CATEGORY C,REGISTERED_USER U,POST P
where P.CATEGORY_ID = C.CATEGORY_ID;

the error message i am getting is
ERROR at line 1:
ORA-00937: not a single-group group function

SQL> desc REGISTERED_USER;
Name Null? Type
----------------------------------------- -------- ----------------------------
USER_ID NOT NULL NUMBER
USERNAME NOT NULL VARCHAR2(10)
PASSWORD NOT NULL VARCHAR2(10)

SQL> desc CATEGORY;
Name Null? Type
----------------------------------------- -------- ----------------------------
CATEGORY_ID NOT NULL NUMBER
CATEGORY_NAME NOT NULL VARCHAR2(255)
USER_ID NUMBER
DESCRIPTION VARCHAR2(255)

SQL> desc POST;
Name Null? Type
----------------------------------------- -------- ----------------------------
POST_ID NOT NULL NUMBER
CATEGORY_ID NUMBER
USER_ID NUMBER
POST_TITLE NOT NULL VARCHAR2(255)
DATE_CREATED NOT NULL DATE
POST_BODY_TEXT NOT NULL VARCHAR2(255)

i used the following commands to create the above tables

create table REGISTERED_USER
(
USER_ID number primary key,
USERNAME varchar2(10) not null,
PASSWORD varchar2(10) not null
);

create table CATEGORY
(
CATEGORY_ID number primary key,
CATEGORY_NAME varchar2(255) not null,
DESCRIPTION varchar2(255),
USER_ID references REGISTERED_USER
);

create table POST
(
POST_ID number primary key,
CATEGORY_ID references CATEGORY,
USER_ID references REGISTERED_USER,
POST_TITLE varchar2(255) not null,
DATE_CREATED date not null,
POST_BODY_TEXT varchar2(255) not null
);

thanks

I forget the formal explanation, but you need to have a GROUP BY clause if you're using certain methods while joining multiple tables.

Try the following:

lol.. did I just "Agree" to a MadCowzz explanation? Wonder if Einstein would be impressed if a n00b told him "Yes, your theory is correct but only if..."

Ya, uh, I left taht one vague on purpose... at least that's my story and I'm sticking to it!
Thanks for the followup Laban