我怎样才能 select 个未回答的问题？

Question

我有一个 table 这样的：

CREATE TABLE `qanda` (
  `id` int(11) UNSIGNED NOT NULL,
  `subject` varchar(150) COLLATE utf8_unicode_ci NOT NULL,
  `body` text COLLATE utf8_unicode_ci NOT NULL,
  `body_html` text COLLATE utf8_unicode_ci NOT NULL,
  `related` int(11) UNSIGNED DEFAULT NULL,
  `type` tinyint(1) NOT NULL,
  `amount` decimal(11,0) DEFAULT NULL,
  `closed` tinyint(1) UNSIGNED DEFAULT NULL,
  `CloserId` varchar(500) COLLATE utf8_unicode_ci NOT NULL,
  `AcceptedAnswer` tinyint(1) DEFAULT NULL,
  `aadate` int(11) UNSIGNED DEFAULT NULL,
  `category` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `keywords` varchar(150) COLLATE utf8_unicode_ci NOT NULL,
  `visibility` tinyint(1) NOT NULL,
  `author_id` int(11) UNSIGNED DEFAULT NULL,
  `editor_id` int(11) UNSIGNED DEFAULT NULL,
  `date_time` int(11) UNSIGNED NOT NULL,
  `edited_at` int(11) UNSIGNED DEFAULT NULL,
  `activated_at` int(11) UNSIGNED DEFAULT NULL,
  `activated_story` enum('سوال شده','ویرایش شده','جواب داده شده','') COLLATE utf8_unicode_ci NOT NULL,
  `activator_id` int(11) UNSIGNED DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

此 table 存储问题和答案。这是我当前的查询，它提供了所有问题的列表并根据投票对它们进行排序：

SELECT *, (SELECT COALESCE(sum(vv.value),0)
           FROM votes vv
           WHERE qanda.id = vv.post_id) AS total_votes
FROM qanda
WHERE type = 0 -- "type=0" means questions
ORDER BY total_votes DESC
LIMIT :j,11;

现在我需要在 WHERE 子句上增加一个条件来排除有答案的问题。我想我需要一个 self-join。但是我不知道如何在 where 子句上写一个 join 。有什么建议吗？

Answer 1

使用 NOT IN 排除已回答的 ID：

SELECT *, (SELECT COALESCE(sum(vv.value),0)
           FROM votes vv
           WHERE qanda.id = vv.post_id) AS total_votes
FROM qanda
WHERE type = 0
AND id NOT IN (SELECT related FROM quanda WHERE type <> 0)
ORDER BY total_votes DESC
LIMIT :j,11;

Answer 2

使用 SELF JOIN，您尝试将问题与任何答案相匹配。如果您没有找到匹配项，您将得到 NULL，因此您将得到没有答案的问题。

SELECT questions.*
FROM qanda questions
LEFT JOIN qanda answers
  ON question.id = answers.related
WHERE questions.type = 0 
  AND answers.id IS NULL

我怎样才能 select 个未回答的问题？

How can I select unanswered questions?

mysql

sql

self-join