How do I get average through ‏multiple tables - sql

Question

I got homework to get ‏average tags of user in album (user_id = x) in the folowing tabels:
>>> CREATE TABLE USERS (ID INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
NAME TEXT NOT NULL);
>>> CREATE TABLE ALBUMS (ID INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
NAME TEXT NOT NULL, CREATION_DATE TEXT NOT NULL,
USER_ID INTEGER REFERENCES USERS(USER_ID) NOT NULL);
>>> CREATE TABLE PICTURES (ID INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
NAME TEXT NOT NULL,
LOCATION TEXT NOT NULL,
CREATION_DATE TEXT NOT NULL,
ALBUM_ID INTEGER REFERENCES ALBUMS(ALBUM_ID) NOT NULL);
>>> CREATE TABLE TAGS (ID INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
PICTURE_ID INTEGER REFERENCES PICTURES(PICTURE_ID) NOT NULL,
USER_ID INTEGER REFERENCES USERS(USER_ID) NOT NULL);";
explenetion:
Each tag is a row in TAGS and it has picture_id, each picture has album_id and each album has user_id, basically i need to count how many times the user is tagged in each album and find the average times that the user is tagged in an album.
I can use this using only: SELECT ? FROM, AVG(), COUNT(), JOIN (INNER, LEFT, RIGHT, FULL JOIN), ON, IN, AND, OR, LIKE, , NOT, (=, != , >, <), IS, DISTINCT, ORDER BY(ASC/DESC), LIMT, OFFSET, and WHERE that means i cannot use GROUP BY
i tried this
SELECT * FROM TAGS INNER JOIN PICTURES ON tags.picture_id = PICTURES.Id where album_id IN (select id from ALBUMS where user_id = x) AND user_id = x;
but it only gives my a table that has all the tags of the user
How can i get the avg tags per album of (user_id = x), is this even possible?

Answer 1

First count how many times the user is tagged in each album and then get the average of these counters:
select
avg(counter) averagetags
from (
select count(t.user_id) counter
from albums a
inner join pictures p on p.album_id = a.id
inner join tags t on t.picture_id = p.id
where t.user_id = ?
group by a.id
)