mysql - SELECT COUNT 与 ORDER BY 混淆

Question

我有一个问题我真的不明白。 查询

SELECT
    f.*,
    ftv.content,
    ftv.updated,
    COUNT(ftv.file_number) AS versions
FROM 
    files as f
INNER JOIN 
    files_text_versions AS ftv
ON 
    ftv.file_number = f.file_number
WHERE 
    f.file_number = '%s'
ORDER BY 
    ftv.updated DESC
LIMIT 1

完全忽略 ORDER BY 语句，但如果我删除

COUNT(ftv.file_number) AS versions

查询运行良好，并按照应有的顺序进行排序。

这是为什么，我该如何解决？

SQL fiddle 有同样的问题但只有一张表:

http://sqlfiddle.com/#!2/c8f124/2

Answer 1

您的count(*) 将查询转换为聚合查询。如果没有 group by，它只会返回一行。虽然在其他数据库中你会得到一个错误，但 MySQL 允许这种语法。

您可以通过添加group by 来解决这个问题。但是，ftv 中的其他两列会出现问题。您可以通过巧妙的聚合获得所需的值:

SELECT f.*,
       substring_index(group_concat(ftv.content order by ftv.updated desc), ',', 1) as content,
       MAX(ftv.updated) as updated,
       COUNT(ftv.file_number) AS versions
FROM files f INNER JOIN 
     files_text_versions AS ftv
     ON ftv.file_number = f.file_number
WHERE f.file_number = '%s'
GROUP BY t.file_number
ORDER BY updated DESC
LIMIT 1;