Mysql 选择要删除的重复行？

我使用它来删除MySql/MariaDB表中的重复记录，因为三列是相同的

DELETE a FROM stest as a, stest as b
WHERE
      (a.facility_id=b.facility_id OR a.facility_id IS NULL AND b.facility_id IS NULL)
  AND (a.inspection_date=b.inspection_date OR a.inspection_date IS NULL AND b.inspection_date IS NULL)
  AND (a.deficiency_tag=b.deficiency_tag OR a.deficiency_tag IS NULL AND b.deficiency_tag IS NULL)
  AND a.recno < b.recno;

我想做的是，在有重复记录的地方，保留长度最大的一个inspection_text列。很可能，检查文本列是相同的，但如果不是，我想删除较小的列

有人能告诉我如何修改上述语句以添加此条件吗

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我也很好奇删除是如何工作的，但是如果我将DELETE a更改为SELECT a.*它不会显示要删除的行，而是显示表中的所有行？

要获取要删除的值，可以使用并与检查文本的max_len值进行内部联接 对于双重许可行，删除长度为max_len的行

   delete from  stest 
   inner join ( 

       select  facility_id, deficiency_tag, inspection_date , max(length( inspection_text)) as  max_len from stest
       where ( facility_id, deficiency_tag, inspection_date ) in ( 

       select facility_id, deficiency_tag, inspection_date 
       from stest
       group by facility_id, deficiency_tag, inspection_date
       having count(*) > 1 
       ) 
       group by  facility_id, deficiency_tag, inspection_date
   ) t  on stest.facility_id = t.facility_id 
              and stest.deficiency_tag = t.deficiency_tag
                and stest.inspection_date = t.inspection_date
                  and length( stest.inspection_text) <> t.max_len 

没有内部联接的同一版本，但其中

   delete from  stest ,  ( 
       select  
            facility_id
          , deficiency_tag
          , inspection_date 
          , max( length( inspection_text) ) as  my_max_len 
       from stest, ( 
          select 
              facility_id
            , deficiency_tag
            , inspection_date 
         from stest
         group by facility_id, deficiency_tag, inspection_date
         having count(*) > 1 
       ) t2 where  stest.facility_id = t2.facility_id and stest.deficiency_tag = t2.deficiency_tag and stest.inspection_date = t2.inspection_date
       group by  facility_id, deficiency_tag, inspection_date
   ) t  where  stest.facility_id = t.facility_id 
              and stest.deficiency_tag = t.deficiency_tag
                and stest.inspection_date = t.inspection_date
                    and length( stest.inspection_text) <> t.my_max_len 

---

如果我将DELETE a更改为SELECT a.*则不会显示要删除的行，而是显示表中的所有行？-您的查询没有执行您认为它正在执行的操作。优先于或。您需要使用另一组括号：ex-其中a.facility_id=b.facility_id或a.facility_id为NULL，b.facility_id为NULL。所有其他WHERE子句都需要相同的更新。您在SELECT语句中看到的结果是100%正确的。此外，请停止使用隐式连接语法。这在25年前被ANSI-92取代。为每个组分配一个用户变量，该变量表示您的工厂、检验、缺陷、长度描述订购的每个组的行号。然后删除行号>1的位置，我在上述语句中得到一个错误，减去我更正的一个语法错误。。1064-您的SQL语法有错误；检查与您的MariaDB服务器版本相对应的手册，了解第2行“内部连接选择设备id、缺陷标记、检查日期、最大值”附近使用的正确语法。我更正了表名。。还是会犯那个错误。我能够在内部联接中执行该语句，并且它可以工作。我看不出错误在哪里。。有什么想法吗？我正在使用5.5.52-MariaDB，如果这有区别的话。。。。也许这个版本不支持内部连接？只是在stest.inspection\u text.和答案之间留一个空格。为mariadb 5.5.52删除的空间我不知道发布日期2016-09-13

   delete from  stest ,  ( 
       select  
            facility_id
          , deficiency_tag
          , inspection_date 
          , max( length( inspection_text) ) as  my_max_len 
       from stest, ( 
          select 
              facility_id
            , deficiency_tag
            , inspection_date 
         from stest
         group by facility_id, deficiency_tag, inspection_date
         having count(*) > 1 
       ) t2 where  stest.facility_id = t2.facility_id and stest.deficiency_tag = t2.deficiency_tag and stest.inspection_date = t2.inspection_date
       group by  facility_id, deficiency_tag, inspection_date
   ) t  where  stest.facility_id = t.facility_id 
              and stest.deficiency_tag = t.deficiency_tag
                and stest.inspection_date = t.inspection_date
                    and length( stest.inspection_text) <> t.my_max_len