Combine search from 2 rows in mysql - mysql

Question

I run a survey where all answers are stored in a separate row in the 'survey' table.
My table looks like this:
(ID,user_id,Q,A)
(1,10,'laundry','oxiclean')
(2,10,'laundry','tide')
(3,10,'laundry','pods')
(4,11,'laundry','spray n wash')
(5,11,'laundry','resolve')
(6,12,'laundry','oxiclean')
(7,13,'laundry','oxiclean')
I now need to pull the count of user id that selected ONLY specific products.
"SELECT *, count(user_id) FROM survey WHERE Q='laundry' GROUP BY a"
the above will give a an overall COUNT but I need to get my count based on users that selected ONLY 'oxiclean' for example. This should return 2.
Or users that selected 'oxiclean' AND 'tide' ONLY.
How do I go about performing this 'combination' of results pulled from different rows?
Thanks a lot!

Answer 1

select user_id from survey group by user_id having count(user_id) = 1
This retrieves a list of users which have only one answer in the survey. Use it as a filter condition:
select q,a,count(user_id)
from survey
where a = 'oxiclean'
and user_id in (select user_id from survey group by user_id having count(user_id) = 1)

Answer 2

You can achieve that using a subquery, for your case it will be something like that :
SELECT *, COUNT(user_id)
FROM survey AS s
WHERE Q = 'laundry'
AND A = 'oxiclean'
AND user_id NOT IN (SELECT user_id FROM survey WHERE Q = s.Q AND A != s.A);
s.Q and s.A refer to the parent field so you don't have to reinject the name a second time.
Downside : the query works only if you want one specific answer.
If you want one query to retrieve the overall count, this one should do the trick :
SELECT A, COUNT(user_id)
FROM (
SELECT A, user_id
FROM survey
WHERE Q='laundry'
GROUP BY user_id
HAVING COUNT(user_id) = 1
) AS t
GROUP BY A
Downside : the query give only answers who have only at least one unique user_id as seen here, and this syntax create a temporary table which is something to avoid for performance reasons.