使用子查询删除重复行

Question

我正在使用 SQL Server 2014 并利用 Microsoft 提供的 AdventureWorks2012 示例数据库。

我正在尝试使用下面的子查询删除重复的行（选项 #2）：

/* 选项 #2：子查询 */

--SELECT * FROM
DELETE SQLPractice.[dbo].[CURRENCY]
WHERE EXISTS (SELECT * 
              FROM
                  (SELECT 
                       NAME,
                       ROW_NUMBER () OVER (PARTITION BY NAME ORDER BY NAME) AS Flag
                   FROM  
                       SQLPractice.[dbo].[CURRENCY]) AS T
              WHERE Flag > 1) 
GO

但它会从表中删除所有行。

但另一个选项 (CTE) 确实只删除了重复的行。

/*** Option #3: CTE ***/ 
;WITH RepFlag AS
(
    SELECT 
        NAME,
        ROW_NUMBER () OVER (PARTITION BY NAME ORDER BY NAME) AS Flag
    FROM 
        SQLPractice.[dbo].[CURRENCY]
)
--SELECT * FROM RepFlag
DELETE RepFlag
WHERE Flag > 1

SELECT * 
FROM SQLPractice.[dbo].[CURRENCY]

请使用以下代码创建您自己的测试表。

/*** REMOVING DUPLICATE ROWS OPTION ***/
-- Creating a table 
SELECT TOP 0 *
INTO [dbo].[CURRENCY]
FROM AdventureWorks2012.Sales.Currency
WHERE NAME LIKE  '%A';

-- inserting duplicate rows 
INSERT [dbo].[CURRENCY]
SELECT * FROM AdventureWorks2012.Sales.Currency
WHERE NAME LIKE  '%A';

/***** SELECTING COUNT OF DUPLICATED ROWS *****/ 

/*** Option #1: "GROUP BY" with "HAVING" ***/ 
SELECT 
    NAME, COUNT(*) AS Qty   
FROM 
    SQLPractice.[dbo].[CURRENCY]
GROUP BY 
    NAME
HAVING 
    COUNT(*) >1
GO

Answer 1

如果您想使用subquery 删除重复的名称，请使用以下方法。

DELETE t
FROM  (SELECT  NAME,ROW_NUMBER () OVER (PARTITION BY NAME ORDER BY NAME) AS Flag
              FROM  SQLPractice.[dbo].[CURRENCY]
            ) t
WHERE t.Flag > 1
GO

您也可以使用 common table expression (CTE) 实现此目的。

;WITH cte_1
AS (SELECT  NAME,ROW_NUMBER () OVER (PARTITION BY NAME ORDER BY NAME) AS Flag
              FROM  SQLPractice.[dbo].[CURRENCY]
            ) 
DELETE FROM cte_1
WHERE Flag > 1

Answer 2

一种可能的方法：

DELETE tt
FROM [your table] tt
   INNER JOIN

    (SELECT NAME, MIN(PK) AS MIN_KEY)
    FROM [your table]
    GROUP BY Name
    HAVING COUNT(*) > 1) dup ON dup.name = tt.name and tt.PK <> dup.MIN_KEY

Answer 3

选项#2 删除所有行，因为EXISTS 中的子查询将始终返回表中所有行的行。 EXISTS 内部的子查询与父查询之间一定存在某种关系。子查询必须根据表的每一行产生不同的结果。当表具有标识列时，使用子查询删除重复行的一种选择是：

DELETE from SQLPractice.[dbo].[CURRENCY]
where identityCol not in ( select min(identityCol) FROM SQLPractice.[dbo].[CURRENCY] GROUP BY NAME)

Answer 4

在您的示例案例中， Row_Number() 不会帮助您解决问题。 因为即使在 CurrencyCode 的主键（候选字段）中，重复行也是相同的

由于您只是将同一行插入到目标表中，因此 ModifiedDate 字段也是相同的。

对于示例案例，您可以应用delete duplicate rows where no primary key exists中描述的解决方案

你可以测试一下，下面的DELETE命令会删除表中的所有行

delete [dbo].[CURRENCY]
from [dbo].[CURRENCY]
inner join (
    select ROW_NUMBER() over (partition by CurrencyCode order by ModifiedDate) rn, CurrencyCode, ModifiedDate from [dbo].[CURRENCY]
) dublicates
    on dublicates.CurrencyCode = [dbo].[CURRENCY].CurrencyCode and
       dublicates.ModifiedDate = [dbo].[CURRENCY].ModifiedDate
where dublicates.rn > 1

例如从教程中，建议使用光标方法 您可以使用以下

DECLARE @Count int
DECLARE @CurrencyCode varchar(10)
DECLARE @ModifiedDate datetime

DECLARE dublicate_cursor CURSOR FAST_FORWARD FOR
SELECT CurrencyCode, ModifiedDate, Count(*) - 1
FROM CURRENCY
GROUP BY CurrencyCode, ModifiedDate
HAVING Count(*) > 1

OPEN dublicate_cursor

FETCH NEXT FROM dublicate_cursor INTO @CurrencyCode, @ModifiedDate, @Count

WHILE @@FETCH_STATUS = 0
BEGIN

SET ROWCOUNT @Count
DELETE FROM CURRENCY WHERE CurrencyCode = @CurrencyCode AND ModifiedDate = @ModifiedDate
SET ROWCOUNT 0

FETCH NEXT FROM dublicate_cursor INTO @CurrencyCode, @ModifiedDate, @Count
END

CLOSE dublicate_cursor
DEALLOCATE dublicate_cursor

Answer 5

With语句只删除重复行，因为它收集所有重复记录，然后执行删除操作。

虽然在您的子查询中您没有指定要删除哪些记录的位置条件，但它应该写成如下：

DELETE SQLPractice.[dbo].[CURRENCY]
WHERE EXISTS  
(
    SELECT * FROM 
    (
        SELECT 
        NAME,
        ID,
        ROW_NUMBER () OVER (PARTITION BY NAME ORDER BY NAME) AS Flag
        FROM SQLPractice.[dbo].[CURRENCY] 
    )   AS T
    WHERE Flag > 1 AND T.ID=[CURRENCY].ID
) 

Answer 6

你可以通过这个查询尝试这个只是重复的记录将被删除我做了这个基于货币重复值它删除所有重复值

delete from test where currency in(select currency from test group by currency having count(*) >1)