Not a single-group group function - sql

Question

I have some tables which basically look as follows:
TBL_USER
user_id - number
user_name - varchar
TBL_STUFF
stuff_id - number
stuff_user_id - number
I want to query for all user information including the number of "stuff" they have. I was trying something like this:
select user_id, user_name, count(stuff_id)
from tbl_user
left outer join tbl_stuff on stuff_user_id = user_id
where user_id = 5;
but I get an error which says "not a single-group group function"
Is there some other way I should be doing this?

Answer 1

Well, you are missing the group function ;-)
Try this:
select user_id, user_name, count(stuff_id)
from tbl_user left outer join tbl_stuff on stuff_user_id = user_id
where user_id = 5
group by user_id, user_name;
The last line is the group by clause that tells Oracle to count all rows with the same user_id and user_name combination.

Answer 2

You could also do it like this:
select
user_id,
user_name,
(
SELECT
COUNT(*)
FROM
tbl_stuff
WHERE
stuff_user_id = tbl_user.user_id
) AS StuffCount,
from
tbl_user
where
user_id = 5;

Answer 3

One of your comments states that you don't want to include all the field present in a GROUP BY clause.
#Arion posted a correlated-sub-query re-factor that gives the same values.
The following query uses a standard (un-correlated) sub-query (inline-view) instead. This is because using this inline-view structure can often perform correlated-sub-query equivilents. But, also, because I find them easier to maintain.
WITH
stuff_count
AS
(
SELECT
stuff_user_id AS user_id,
COUNT(*) AS val
FROM
tbl_stuff
GROUP BY
stuff_user_id
)
SELECT
tbl_user.user_id,
tbl_user.user_name,
stuff_count.val
FROM
tbl_user
LEFT JOIN
stuff_count
ON stuff_count.user_id = tbl_user.user_id
WHERE
tbl_user.user_id = 5;
NOTE: When the plan is generated, it only runs the sub-query for the user_id's necessary, not the whole table ;)