mysql - 无法计算连接表 mySQL 的结果

Question

我关注了其他 3 个 SO 答案，他们说它有效..但这里不是...这是我的问题，我有这个问题:

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

这是查询结果:

到这里为止，它是完美的..现在我需要知道返回了多少行...我将查询更改为:

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

出现这个错误:

为什么我数不清我得到了多少行？

更新:我也试过做这个查询:

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

得到这个结果:

Answer 1

你可以这样改:

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
)