Question

我使用了以下查询，但显示错误。

SELECT 
    id as number,id,parentid,videoid,subject,name,created,message 
FROM 
    umqlh_hdflv_comments 
WHERE 
    `parentid` = '0' 
    AND `published` = '1' 
    AND `videoid` = '8' 
ORDER BY 
    number DESC,
    parentid 
UNION ( 
    SELECT 
        parentid as number,id,parentid,videoid,subject,name,created,message 
    FROM 
        umqlh_hdflv_comments 
    WHERE 
        `parentid` != '0' 
        AND `published` = '1' 
        AND `videoid` = '8'
    )

MySQL说：文档

#1221 - Incorrect usage of UNION and ORDER BY

如何解决此问题？

Answer 1

ORDER BY需要在完整的sql语句结束时应用

SELECT 
    id as number,
    id,
    parentid,
    videoid,
    subject,
    name,
    created,
    message 
FROM 
    umqlh_hdflv_comments 
WHERE 
    `parentid` = '0' 
    AND `published` = '1' 
    AND `videoid` = '8' 
 UNION ( 
    SELECT 
        parentid as number,
        id,
        parentid,
        videoid,
        subject,
        name,
        created,
        message 
    FROM 
        umqlh_hdflv_comments 
    WHERE 
        `parentid` != '0' 
        AND `published` = '1' 
        AND `videoid` = '8'
    )
    ORDER BY number DESC,parentid

Reference

Answer 2

从语法上讲，您需要将订单移动到查询结尾。除了优化之外，例如＆＃34;限制＆＃34;在联合中订购其中一个零件是没有意义的，因为无论如何都不能保证对结果进行分类。

SELECT id as number
      ,id
      ,parentid
      ,videoid
      ,subject
      ,name
      ,created,message 
FROM umqlh_hdflv_comments 
WHERE `parentid` = '0' 
  AND `published` = '1' 
  AND `videoid` = '8' 
UNION 
SELECT parentid as number
     ,id
     ,parentid
     ,videoid
     ,subject
     ,name
     ,created,message 
FROM umqlh_hdflv_comments 
WHERE `parentid` != '0' 
 AND `published` = '1' 
 AND `videoid` = '8'
ORDER BY number DESC,parentid

但是，此查询也可以表示为：

SELECT case when parentid = 0 then id else parentid end as number
      ,id
      ,parentid
      ,videoid
      ,subject
      ,name
      ,created,message 
FROM umqlh_hdflv_comments 
WHERE `published` = '1' 
  AND `videoid` = '8' 
ORDER BY number DESC,parentid

如果parentid可以为null，则需要在where子句中添加parentid不为null。此外，如果行不是唯一的（我假设这个，因为列被命名为id），您需要为查询添加distinct以使其等效。

UNION和ORDER By MySql

2 个答案: