从表中删除重复记录 - SQL 查询

Question

我只需要从表中删除重复行，就像表中有 3 个重复行一样，我的查询将从 3 个重复行中删除 2 行。

我怎样才能得到这个？请帮我。

Answer 1

请尝试以下查询，它一定会满足您的目标

SET ROWCOUNT 1
DELETE test
FROM test a
WHERE (SELECT COUNT(*) FROM test b WHERE b.name = a.name) > 1
WHILE @@rowcount > 0
  DELETE test
  FROM test a
  WHERE (SELECT COUNT(*) FROM test b WHERE b.name = a.name) > 1
SET ROWCOUNT 0

，其中 test 是您的表名称

Answer 2

这在 SQL Server 中有效，尽管它不是单个语句：

Declare @cnt int; 
Select @cnt=COUNT(*) From DupTable Where (Col1=1);  -- Assumes you are trying to delete the duplicates where some condition (e.g. Col1=1) is true.
Delete Top (@cnt-1) From DupTable

它也不需要任何额外假设（例如存在使每行唯一的另一列）。毕竟，桑塔努确实说过行是重复的，而不仅仅是一列。

然而，在我看来，正确的答案是获得真正的表结构。也就是说，向该表添加一个 IDENTITY 列，以便您可以使用单个 SQL 命令来完成您的工作。像这样：

ALTER TABLE dbo.DupTable ADD
    IDCol int NOT NULL IDENTITY (1, 1)
GO

然后删除就很简单了：

DELETE FROM DupTable WHERE IDCol NOT IN 
   (SELECT MAX(IDCol) FROM DupTable GROUP BY Col1, Col2, Col3)

Answer 3

DELETE FROM Table t1, Table t2 WHERE t1.colDup = t2.colDup AND t1.date < t2.date

将从Table（在colDup列）中删除除最旧的（即lowset date）之外的所有重复行。

Answer 4

DELETE FROM `mytbl`
    INNER JOIN (
        SELECT 1 FROM `mytbl`
        GROUP BY `duplicated_column` HAVING COUNT(*)=2
    ) USING(`id`)

编辑：

我的错，上面的查询不起作用。

假设表结构：

id int auto_increment

num int # <-- 这是具有重复值的列

以下查询将在 MySQL 中运行（我检查过）：

DELETE `mytbl` FROM `mytbl` 
    INNER JOIN 
    (
        SELECT `num` FROM `mytbl`
        GROUP BY `num` HAVING COUNT(*)=2
    ) AS `tmp` USING (`num`)

该查询将删除 num 列中具有 2 个（不能超过或其他）重复值的行。

编辑（再次）：

我建议在 num 列上添加一个键。

编辑(#3)：

如果作者想要删除重复的行，以下内容应该适用于MySQL（它对我有用）：

DELETE `delete_duplicated_rows` FROM `delete_duplicated_rows`
    NATURAL JOIN (
        SELECT *
        FROM `delete_duplicated_rows`
        GROUP BY `num1` HAVING COUNT(*)=2
    ) AS `der`

假设表结构是：

CREATE TABLE `delete_duplicated_rows` (
  `num1` tinyint(4) DEFAULT NOT NULL,
  `num2` tinyint(4) DEFAULT NOT NULL
) ENGINE=MyISAM;

Answer 5

如果您有要删除的行的 ID，那么...

DELETE FROM table WHERE id IN (1, 4, 7, [id numbers to delete...])

Answer 6

我认为每个表都有唯一的标识符。
因此，如果它存在，那么您可以编写以下查询：
从 Table1 t1 中删除 Table1，其中 2 >=（从 Table1 中选择 count(id)，其中 dupColumn = t1.dupColumn）并且
t1.id 不在（从 Table1 中选择 max (id)，其中 dupColumn = t1.dupColumn）

OOps。看来只能使用第二个过滤器
从 Table1 t1 中删除 Table1，其中
t1.id 不在（从 Table1 中选择 max (id)，其中 dupColumn = t1.dupColumn）

Answer 7

  -- Just to demonstrates Marks example          
    . 
        -- START === 1.0.dbo..DuplicatesTable.TableCreate.sql
    /****** Object:  Table [dbo].[DuplicatesTable] 
        Script Date: 03/29/2010 21:24:02 ******/
      IF EXISTS (SELECT * FROM sys.objects 
     WHERE 
object_id = OBJECT_ID(N'[dbo].[DuplicatesTable]') 
AND type in (N'U'))
        DROP TABLE [dbo].[DuplicatesTable]
    GO

    /****** Object:  Table [dbo].[DuplicatesTable]    
Script Date: 03/29/2010 21:24:02 ******/
    SET ANSI_NULLS ON
    GO

    SET QUOTED_IDENTIFIER ON
    GO

    CREATE TABLE [dbo].[DuplicatesTable](
        [ColA] [varchar](10) NOT NULL, -- the name of the DuplicatesTable
        [ColB] [varchar](10) NULL,  -- the description of the e DuplicatesTable 
     ) 


    /* 
    <doc> 
    Models a DuplicatesTable for 
    </doc>

    */


    GO


    --============================================================ DuplicatesTable START
    declare @ScriptFileName varchar(2000)
    SELECT @ScriptFileName = '$(ScriptFileName)'
    SELECT @ScriptFileName + ' --- DuplicatesTable START =========================================' 
    declare @TableName varchar(200)
    select @TableName = 'DuplicatesTable'

    SELECT 'SELECT name from sys.tables where name =''' + @TableName + ''''
    SELECT name from sys.tables 
    where name = @TableName

    DECLARE @TableCount INT 
    SELECT @TableCount  = COUNT(name ) from sys.tables 
        where name =@TableName

    if @TableCount=1
    SELECT ' DuplicatesTable PASSED. The Table ' + @TableName + ' EXISTS ' 
    ELSE 
    SELECT ' DuplicatesTable FAILED. The Table ' + @TableName + ' DOES NOT EXIST ' 
    SELECT @ScriptFileName + ' --- DuplicatesTable END =========================================' 
    --============================================================ DuplicatesTable END

    GO


    -- END ===  1.0.dbo..DuplicatesTable.TableCreate.sql

    . 
    -- START === 1.1..dbo..DuplicatesTable.TableInsert.sql

    BEGIN TRANSACTION;
    INSERT INTO [dbo].[DuplicatesTable]([ColA], [ColB])
    SELECT   N'ColA', N'ColB' UNION ALL
    SELECT N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA', N'ColB' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1' UNION ALL
    SELECT  N'ColA1', N'ColB1'
    COMMIT;
    RAISERROR (N'[dbo].[DuplicatesTable]: Insert Batch: 1.....Done!', 10, 1) WITH NOWAIT;
    GO


    -- END ===  1.1..dbo..DuplicatesTable.TableInsert.sql

    . 
    -- START === 2.0.RemoveDuplicates.Script.sql
    ALTER TABLE dbo.DuplicatesTable ADD
            DuplicatesTableId int NOT NULL IDENTITY (1, 1)
    GO

    -- Then the delete is trivial:
    DELETE FROM dbo.DuplicatesTable WHERE DuplicatesTableId NOT IN 
         (SELECT MAX(DuplicatesTableId) FROM dbo.DuplicatesTable GROUP BY ColA , ColB)

         Select * from DuplicatesTable ;  
    -- END ===  2.0.RemoveDuplicates.Script.sql