Question

为什么我在这个查询中遇到这个错误？

SELECT distinct fileID 
FROM ( SELECT fileID from file order by fileID desc limit 30) as R1 
UNION (SELECT fileID from visit order by counter desc limit 30 ) 
WHERE status = 1 order by rand() LIMIT 10

错误：

您的SQL语法有错误;检查手册对应于您的MySQL服务器版本，以便使用正确的语法靠近'WHERE status = 1 order by rand（）LIMIT 10'在第1行

我想要的是选择前30个查看次数最多的文件和前30个最新文件，然后从文件状态= 1中随机选择限制10个。

Answer 1

将您的两个查询放入子查询中的UNION中的最新文件和大多数访问，然后通过RAND重新排序生成的派生表，并将结果限制为10.

注意：

你声明你想要选择最近访问过的文件..但是“柜台”的排序实际上选择访问次数最多的文件..而不是最近的文件。

SELECT DISTINCT fileID 
FROM (
  SELECT file.fileID 
    FROM file 
    WHERE file.status = 1 
    ORDER BY file.fileID DESC 
    LIMIT 30
  UNION ALL
  SELECT visit.fileID 
    FROM visit 
      JOIN file ON file.fileID = visit.fileID 
    WHERE file.status = 1 
    ORDER BY visit.counter DESC 
    LIMIT 30 
) dt
ORDER BY RAND() 
LIMIT 10

作为对OP评论的反应，我在其中一个选项中添加了JOIN，以检查与访问相关的文件是否具有status = 1。

另请注意：

要使此查询快速执行大量数据，您应在字段file.status和visit.counter上添加索引。如果fileID不是这些表的键/索引，那么您也应该在此字段上添加索引。

visit.file_id不是UNIQUE，因此上述查询可能在第二个子查询的子结果中具有相同的file_id（重复项将被最终DISTINCT删除但是意味着第二个LIMIT 30不能按预期工作。可能的纠正：

SELECT DISTINCT fileID 
FROM (
  SELECT file.fileID 
    FROM file 
    WHERE file.status = 1 
    ORDER BY file.fileID DESC 
    LIMIT 30
  UNION ALL
  SELECT visit.fileID 
    FROM visit 
      JOIN file ON file.fileID = visit.fileID 
    WHERE file.status = 1 
    GROUP BY visit.fileId
    ORDER BY MAX(visit.counter) DESC 
    LIMIT 30 
) dt
ORDER BY RAND() 
LIMIT 10

Answer 2

您想对此查询做什么？我认为，解析错误更多。

我不知道MySQL是否接受了...... ..如果是你想做的......

    SELECT distinct fileID FROM (( SELECT fileID from file order by
fileID desc limit 10) as R1 UNION (SELECT fileID from visit order by
counter desc limit 10 ) as R2) WHERE R1.status = 1 order by R1.rand() LIMIT 10

Answer 3

试试这个，您需要括号来关闭选择。

SELECT DISTINCT fileid
FROM   ((SELECT fileid, status 
         FROM   FILE
         ORDER  BY fileid DESC
         LIMIT  10)
        UNION
        (SELECT fileid, status 
         FROM   visit
         ORDER  BY counter DESC
         LIMIT  10)) AS T
WHERE  status = 1
ORDER  BY Rand()
LIMIT  10

MySQL Union错误

3 个答案: