Question

我的网站有一个帖子数据库。如果用户已经投票并且他们已经返回投票类型，我试图在查询内部返回。

这是我的查询

    SELECT p.*, 
        (SELECT Count(id) 
        FROM   comments AS c 
        WHERE  c.post = p.id) AS commentCount,
        v.type AS vote_type
   FROM   posts AS p 
   LEFT JOIN votes AS v
   ON v.post_id = p.id
   WHERE v.user_id = 1
   ORDER  BY timestamp DESC

虽然这确实会返回带有正确数据的字段vote_type，但它也会将结果限制为对该用户进行投票的结果。因此，它不会显示所有帖子，而只显示用户投票的帖子？如果没有找到结果，有没有办法让字段默认为0？

Answer 1

<强>问题：

尽管您使用了LEFT JOIN WHERE条款v.user_id = 1。所以其他所有人（user_id都是null）都没有被提取。

<强>解决方案：

在WHERE子句中添加一个条件：

SELECT p.*, 
        (SELECT Count(id) 
        FROM   comments AS c 
        WHERE  c.post = p.id) AS commentCount,
        v.type AS vote_type
   FROM   posts AS p 
   LEFT JOIN votes AS v 
   ON v.post_id = p.id
   WHERE (v.user_id = 1 OR v.user_id IS NULL)
   ORDER  BY timestamp DESC

或

您也可以在JOIN本身
中指定它
SELECT p.*, (SELECT Count(id) FROM comments AS c WHERE c.post = p.id) AS commentCount, v.type AS vote_type FROM posts AS p LEFT JOIN votes AS v ON v.post_id = p.id AND v.user_id = 1 ORDER BY timestamp DESC

如果没有找到结果，左连接默认为零？

1 个答案: