Question

我有三个表：stories，story_types和comments

以下查询检索stories表中的所有记录，获取他们的story_types以及与每个故事相关的评论数量：

         SELECT s.id AS id,
                s.story_date AS datetime,
                s.story_content AS content,
                t.story_type_label AS type_label,
                t.story_type_slug AS type_slug,
                COUNT(c.id) AS comment_count
           FROM stories AS s
LEFT OUTER JOIN story_types AS t ON s.story_type_id = t.id
LEFT OUTER JOIN comments AS c ON s.id = c.story_id 
       GROUP BY s.id;

现在我想做的只是从stories WHERE s.id = 1（这是主键）检索记录。我尝试了以下内容，但它仍然返回所有记录：

         SELECT s.id AS id,
                s.story_date AS datetime,
                s.story_content AS content,
                t.story_type_label AS type_label,
                t.story_type_slug AS type_slug,
                COUNT(c.id) AS comment_count
           FROM stories AS s
LEFT OUTER JOIN story_types AS t ON s.story_type_id = t.id
                AND s.id = 1
LEFT OUTER JOIN comments AS c ON s.id = c.story_id 
       GROUP BY s.id;

我最后还尝试了WHERE子句，这会引发错误。

在这种情况下，有人能为这样的条件指出正确的语法吗？

我正在使用MySQL 5.1.47。感谢。

Answer 1

我猜你把GROUP BY放在了WITH后面，这是非法的。有关MySQL中的SELECT语法，请参阅此reference。试试这个：

SELECT
    s.id AS id,
    s.story_date AS datetime,
    s.story_content AS content,
    t.story_type_label AS type_label,
    t.story_type_slug AS type_slug,
    COUNT(c.id) AS comment_count
FROM
    stories AS s
    LEFT JOIN story_types AS t ON s.story_type_id = t.id
    LEFT JOIN comments AS c ON s.id = c.story_id 
WHERE
    s.id = 1
GROUP BY
    s.id;

编者注：我重新格式化了代码以突出显示查询结构

Answer 2

对接受的答案发表评论：

对于我而言，这个WHERE将进入第二个JOIN
并不直观

这只是为了概述正确的代码格式如何增强理解。以下是我通常格式化SQL的方法：

SELECT 
  s.id               AS id,
  s.story_date       AS datetime,
  s.story_content    AS content,
  t.story_type_label AS type_label,
  t.story_type_slug  AS type_slug,
  COUNT(c.id)        AS comment_count
FROM 
  stories                  AS s
  LEFT JOIN story_types    AS t ON t.id = s.story_type_id 
  LEFT OUTER JOIN comments AS c ON s.id = c.story_id 
WHERE
  s.id = 1
GROUP BY
  s.id;

WHERE不在第二个连接上。在SELECT语句中只允许一个WHERE子句，它始终是顶级的。

PS：还要注意，在许多数据库引擎中（除了MySQL），使用GROUP BY子句然后选择列而不通过MIN()，MAX()或{等函数聚合它们是非法的。 {1}}。恕我直言，这是一种糟糕的风格和不良习惯。

使用LEFT OUTER JOIN和WHERE进行MySQL查询

2 个答案: