Question

以下查询正在做我需要的事情：

SELECT assign.from_uid, assign.aid, assign.message, curriculum.asset, 
       curriculum.title, curriculum.description 
FROM assignment assign 
INNER JOIN curriculum_topics_assets curriculum 
   ON assign.nid = curriculum.asset 
WHERE assign.to_uid = 13 AND assign.status = 1 
GROUP BY assign.from_uid, assign.to_uid, assign.nid 
ORDER BY assign.created DESC

现在我需要得到结果的总行数。例如，如果它显示5行，则o / p应该类似于我预期的o / p。我试过的查询如下：

SELECT count(description) FROM assignment assign 
INNER JOIN curriculum_topics_assets curriculum ON assign.nid = curriculum.asset 
WHERE assign.to_uid = 13 AND assign.status = 1 
GROUP BY assign.from_uid, assign.to_uid, assign.nid 
ORDER BY assign.created DESC

我预期的o / p：

count(*)
---------
5

我当前的o / p：

count(*)
---------
6
2
5
6
6

Answer 1

最简单的解决方案是

将您的初始GROUP BY查询放入子选择
选择从此子选择中检索的行数

SQL语句

SELECT COUNT(*)
FROM   (
          SELECT  assign.from_uid 
          FROM    assignment assign 
                  INNER JOIN curriculum_topics_assets curriculum ON assign.nid = curriculum.asset 
          WHERE   assign.to_uid = 13 
                  AND assign.status = 1 
          GROUP BY 
                  assign.from_uid
                  , assign.to_uid
                  , assign.nid 
       ) q

编辑 - 为什么原始查询不返回所需的结果

它已经准备好获取正确结果所需的内容

没有分组的查询会返回25条记录的结果集（6 + 2 + 5 + 6 + 6）
从这25条记录中，您有from_uid, to_uid, nid

现在你不想计算每个组合有多少记录（正如你在你的例子中所做的那样），但是有多少独特的（不同于任何人？）组合。

对此的一个解决方案是我提出的子选择，但使用DISTINCT子句的等效语句可能更全面。

SELECT  COUNT(*)
FROM    (
          SELECT  DISTINCT assign.from_uid
                  , assign.to_uid
                  , assign.nid
          FROM    assignment assign 
                  INNER JOIN curriculum_topics_assets curriculum ON assign.nid = curriculum.asset 
          WHERE   assign.to_uid = 13 
                  AND assign.status = 1 
        ) q

请注意，我个人的偏好是GROUP BY解决方案。

Answer 2

要获取查询的行数，请执行以下操作：

SELECT COUNT(*) as RowCount FROM (--insert other query here--) s

在你的例子中：

SELECT COUNT(*) as RowCount FROM (SELECT a.from_uid
  FROM assignment a 
  INNER JOIN curriculum_topics_assets c ON a.nid = c.asset 
  WHERE a.to_uid = 13 
    AND a.status = 1 
  GROUP BY a.from_uid, a.to_uid, a.nid 
  ) s

请注意，我删除了对行数没有影响的内容，以使查询运行得更快。

Answer 3

您应该使用COUNT(*)代替count(description)。请看：http://www.mysqlperformanceblog.com/2007/04/10/count-vs-countcol/

查询输出与预期输出不同

3 个答案:

SQL语句

编辑 - 为什么原始查询不返回所需的结果