percent match within the same table
You need to have a subquery (q1) to count how many answers of every user coincide with those of user 10 (number_of_coinciding_answers). Then, have a second subquery (q2) that counts the number_of_answers. 

Then compute 100.0 * number_of_coinciding_answers / number_of_answers:

Everything put together just looks like:

SELECT
    q1.user, 
    100.0 * q1.number_of_coinciding_answers / q2.number_of_answers AS percent
FROM
    (SELECT
        t.user, 
        count(good.answer) AS number_of_coinciding_answers
    FROM
        t 
        JOIN t AS good ON good.question = t.question 
                      AND good.user = 10 
                      AND good.answer = t.answer
    WHERE
        t.user &lt;&gt; 10
    GROUP BY
        t.user
    ) AS q1
    CROSS JOIN
    (SELECT
        count(*) AS number_of_answers
    FROM
        t
    WHERE 
        t.user = 10
    ) AS q2  
ORDER BY
    q1.user ;


You'll get


user |  percent
---: | -------:
  11 | 50.00000
  12 | 50.00000


Note that the JOIN condition is taking care of just finding matching answers between the current user and the reference user (user 10).

Note also that the numbers retrieived are 50% and 50%, which are the correct ones, as pointed out by Rick James.



You can write it in a slightly more compact fashion (that MySQL actually seems to handle better) by using:

SELECT
    q1.user, 
    100.0 * q1.number_of_coinciding_answers / 
          (SELECT count(*) FROM  t WHERE  t.user = 10) AS percent
FROM
    (SELECT
        t.user, 
        count(good.answer) AS number_of_coinciding_answers
    FROM
        t 
        JOIN t AS good ON good.question = t.question 
                      AND good.user = 10 
                      AND good.answer = t.answer
    WHERE
        t.user &lt;&gt; 10
    GROUP BY
        t.user
    ) AS q1
ORDER BY
    q1.user ;




All these queries are helped by having a multi-column covering index on columns (user, question, answer). That is, in MySQL, you'd define your table 

CREATE INDEX idx_t_uqa 
    ON t (user, question, answer) ;


You can see a more step-by-step approach and all the settings at dbfiddle here