mysql 使用 where 子句删除重复项

Question

我正在尝试删除具有相同 hid 值的重复记录。

这是Fiddle：

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=86e8ed00cf0a496da490eae5d7aae093

Table product_match_unmatches:

ID  hid flag
1   1   1
2   1   1
3   2   1
4   2   1
5   1   2
6   2   2
7   2   2
8   1   1
9   1   1
10  2   1

现在我想从 table 中删除重复项 hid 但仅针对 flag = 1。

此查询将删除所有 duplicates 除了最近的一个，但不考虑标志值：

DELETE pmu1
FROM dmf_product_match_unmatches as pmu1
LEFT JOIN ( SELECT MAX(ID) as ID, hid
            FROM dmf_product_match_unmatches as pmu2
            GROUP BY hid) pmu3 USING (ID, hid)
WHERE pmu3.ID IS NULL;

我试图在上面的查询中添加 where 子句 flag = 1，但这没有产生预期的结果。

DELETE pmu1
FROM dmf_product_match_unmatches as pmu1
LEFT JOIN ( SELECT MAX(ID) as ID, hid
            FROM dmf_product_match_unmatches as pmu2
            where flag = 1
            GROUP BY hid
          ) pmu3 USING (ID, hid)
WHERE pmu3.ID IS NOT NULL;

要求的输出是：

ID  hid flag
5   1   2
6   2   2
7   2   2
9   1   1
10  2   1

Answer 1

你需要

DELETE t1
FROM dmf_product_match_unmatches t1
JOIN dmf_product_match_unmatches t2 USING (hid, flag)
WHERE flag = 1 
  AND t1.id < t2.id;

?

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=a5e9e95335573ebedd45cdcd577b5602

Answer 2

使用row_number

DELETE pmu1
FROM dmf_product_match_unmatches pmu1
JOIN (select id,
         row_number() over(partition by hid order by id desc) rn
       from dmf_product_match_unmatches 
       where flag = 1
     ) as pmu3 ON pmu1.ID = pmu3.ID
WHERE pmu3.rn > 1;

Answer 3

在 sql 服务器中，您可以使用 EXCEPT 设置运算符：

declare @flag int = 1

delete dmf_product_match_unmatches
where id in (

    select id
    from dmf_product_match_unmatches
    where flag = @flag

    except

    select max(id) id
    from dmf_product_match_unmatches
    where flag = @flag
    group by hid, flag
)

在mysql中你可以使用NOT EXISTS

declare @flag int = 1

delete d1
from dmf_product_match_unmatches d1
where flag = @flag
and not exists (
    select max(id) id
    from dmf_product_match_unmatches  d2
    group by hid, flag
    having d1.id = max(d2.id)       
)

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