无法计算联接表mySQL的结果

Question

i followed 3 others SO answers, and they say it works.. but here not... here is my problem, i have this QUERY:

SELECT
    q.*
FROM relacionamento AS r
INNER JOIN questoes AS q ON r.idquestao = q.id
WHERE tabela = 'disciplina'
GROUP BY q.id

This is there result of query:

Until here, it was perfect.. now i need know how much rows returned... i changed the query to this:

SELECT
    COUNT(q.*)
FROM relacionamento AS r
INNER JOIN questoes AS q ON r.idquestao = q.id
WHERE tabela = 'disciplina'
GROUP BY q.id

And got this error:

Why i cant count how much rows did i get?

UPDATE: I did try do this query also:

SELECT
    COUNT(*)
FROM relacionamento AS r
INNER JOIN questoes AS q ON r.idquestao = q.id
WHERE tabela = 'disciplina'
GROUP BY q.id

And get this as result:

Answer 1

You can change it like this:

SELECT COUNT(*) FROM (
    SELECT
        q.*
    FROM relacionamento AS r
    INNER JOIN questoes AS q ON r.idquestao = q.id
    WHERE tabela = 'disciplina'
    GROUP BY q.id
)

Answer 2

Your query is probably better written as:

SELECT q.*
FROM questoes q
WHERE EXISTS (SELECT 1
              FROM relacionamento r
              WHERE r.idquestao = q.id AND r.tabela = 'disciplina'
             );

This should be more efficient than your query, if you have an index on relacionamento(idquestao, tablea) (which you would want anyway for the join ). And, it doesn't use select * with group by , which just generally looks wrong.

Then, to get the count, you can just do count(*) :

SELECT COUNT(*)
FROM questoes q
WHERE EXISTS (SELECT 1
              FROM relacionamento r
              WHERE r.idquestao = q.id AND r.tabela = 'disciplina'
             );