整蛊的MS Access SQL查询来删除多余的重复记录

Question

我有以下形式的Access表（我简化了一点）

ID   AutoNumber  Primary Key 
SchemeName Text (50) 
SchemeNumber Text (15) 

这包含了一些数据，例如...

ID   SchemeName   SchemeNumber 
-------------------------------------------------------------------- 
714   Malcolm    ABC123 
80   Malcolm    ABC123 
96   Malcolms Scheme  ABC123 
101   Malcolms Scheme  ABC123 
98   Malcolms Scheme  DEF888 
654   Another Scheme  BAR876 
543   Whatever Scheme  KJL111 
etc... 

现在。我想在相同的SchemeNumber下删除重复的名称。但是我想离开该Scheme编号的SchemeName最长的记录。 如果有长度相同的重复记录，那么我只想留下一个，即最低的ID（但任何一个都会真的）。从上面的例子中，我想删除ID 714,80和101（仅留下96）。

我认为这样会比较容易实现，但它变成了一个噩梦！感谢您的任何建议。我知道我可以循环编程，但我宁愿有一个DELETE查询。

Answer 1

看是否有此查询返回你想保留的行：

SELECT r.SchemeNumber, r.SchemeName, Min(r.ID) AS MinOfID 
FROM 
    (SELECT 
     SchemeNumber, 
     SchemeName, 
     Len(SchemeName) AS name_length, 
     ID 
    FROM tblSchemes 
    ) AS r 
    INNER JOIN 
    (SELECT 
     SchemeNumber, 
     Max(Len(SchemeName)) AS name_length 
    FROM tblSchemes 
    GROUP BY SchemeNumber 
    ) AS w 
    ON 
     (r.SchemeNumber = w.SchemeNumber) 
     AND (r.name_length = w.name_length) 
GROUP BY r.SchemeNumber, r.SchemeName 
ORDER BY r.SchemeName; 

如果是这样，将其保存为qrySchemes2Keep。然后创建一个DELETE查询，以放弃在qrySchemes2Keep中找不到其ID值的tblSchemes中的行。

DELETE 
FROM tblSchemes AS s 
WHERE Not Exists (SELECT * FROM qrySchemes2Keep WHERE MinOfID = s.ID); 

只是要小心，如果以后使用Access'查询设计器进行修改，即删除查询，它可能‘有益’的SQL转换为这样的事情：

DELETE s.*, Exists (SELECT * FROM qrySchemes2Keep WHERE MinOfID = s.ID) 
FROM tblSchemes AS s 
WHERE (((Exists (SELECT * FROM qrySchemes2Keep WHERE MinOfID = s.ID))=False)); 

Answer 2

DELETE FROM Table t1 
WHERE EXISTS (SELECT 1 from Table t2 
      WHERE t1.SchemeNumber = t2.SchemeNumber 
      AND Length(t2.SchemeName) > Length(t1.SchemeName) 
) 

取决于你的RDBMS你可以使用功能不同长度（甲骨文 - 长度，MySQL的 - 长度，SQL服务器 - LEN）

Answer 3

试试这个：

Select * From Table t 
    Where Len(SchemeName) < 
     (Select Max(Len(Schemename)) 
     From Table 
     Where SchemeNumber = t.SchemeNumber) 
    And Id > 
     (Select Min (Id) 
     From Table 
     Where SchemeNumber = t.SchemeNumber 
      And SchemeName = t.SchemeName) 

或本:, ...

Select * From Table t 
    Where Id > 
     (Select Min(Id) From Table 
     Where SchemeNumber = t.SchemeNumber 
     And Len(SchemeName) < 
      (Select Max(Len(Schemename)) 
      From Table 
      Where SchemeNumber = t.SchemeNumber)) 

如果其中任一选择应删除的记录，只需将其更改为删除

Delete 
    From Table t 
    Where Len(SchemeName) < 
     (Select Max(Len(Schemename)) 
     From Table 
     Where SchemeNumber = t.SchemeNumber) 
    And Id > 
     (Select Min (Id) 
     From Table 
     Where SchemeNumber = t.SchemeNumber 
      And SchemeName = t.SchemeName) 

或使用第二建筑：

Delete From Table t Where Id > 
    (Select Min(Id) From Table 
    Where SchemeNumber = t.SchemeNumber 
    And Len(SchemeName) < 
     (Select Max(Len(Schemename)) 
     From Table 
     Where SchemeNumber = t.SchemeNumber)) 

Answer 4

delete ShortScheme 
from Scheme ShortScheme 
join Scheme LongScheme 
    on ShortScheme.SchemeNumber = LongScheme.SchemeNumber 
    and (len(ShortScheme.SchemeName) < len(LongScheme.SchemeName) or (len(ShortScheme.SchemeName) = len(LongScheme.SchemeName) and ShortScheme.ID > LongScheme.ID)) 

（SQL服务器有味）

现在更新，包括指定的领带分辨率。虽然，在两个查询中可以获得更好的性能：首先使用较短的名称删除计划（如我的原始查询），然后返回并删除名称长度相同的较高ID。

Answer 5

如果你的平台支持排名函数和公用表表达式：

with cte as (
    select row_number() 
    over (partition by SchemeNumber order by len(SchemeName) desc) as rn 
    from Table) 
delete from cte where rn > 1; 

Answer 6

我会为此在多个步骤。一步完成的大量删除操作让我感到非常紧张 - 如果您犯了一个错误，该怎么办？没有sql'undo'语句。

-- Setup the data 
DROP Table foo; 
DROP Table bar; 
DROP Table bat; 
DROP Table baz; 
CREATE TABLE foo (
    id int(11) NOT NULL, 
    SchemeName varchar(50), 
    SchemeNumber varchar(15), 
    PRIMARY KEY (id) 
); 

insert into foo values (714, 'Malcolm', 'ABC123'); 
insert into foo values (80, 'Malcolm', 'ABC123'); 
insert into foo values (96, 'Malcolms Scheme', 'ABC123'); 
insert into foo values (101, 'Malcolms Scheme', 'ABC123'); 
insert into foo values (98, 'Malcolms Scheme', 'DEF888'); 
insert into foo values (654, 'Another Scheme ', 'BAR876'); 
insert into foo values (543, 'Whatever Scheme ', 'KJL111'); 

-- Find all the records that have dups, find the longest one 
create table bar as 
    select max(length(SchemeName)) as max_length, SchemeNumber 
    from foo 
    group by SchemeNumber 
    having count(*) > 1; 

-- Find the one we want to keep 
create table bat as 
    select min(a.id) as id, a.SchemeNumber 
    from foo a join bar b on a.SchemeNumber = b.SchemeNumber 
     and length(a.SchemeName) = b.max_length 
    group by SchemeNumber; 

-- Select into this table all the rows to delete 
create table baz as 
    select a.id from foo a join bat b where a.SchemeNumber = b.SchemeNumber 
     and a.id != b.id; 

这会为您提供一个只包含要删除的行的记录的新表。

现在检查这些，并确保它们只包含要删除的行。通过这种方式，您可以确保在执行删除操作时，您确切地知道将会发生什么。它也应该很快。

然后，当您准备好时，使用此命令使用此命令删除行。

delete from foo where id in (select id from baz); 

这似乎是更多的工作，因为不同的表，但它更安全，可能与其他方式一样快。另外，您可以在任何步骤中停止并确保在执行任何实际删除之前数据是您想要的。