在MySQL中查找重复值-Java 学习之路

648

我有一个包含varchar列的表，我想在此列中找到所有具有重复值的记录 . 我可以用什么来查找重复的最佳查询？

22 回答

尝试使用此查询：

SELECT name, COUNT(*) value_count FROM company_master GROUP BY name HAVING value_count > 1;

回复于 2024-04-29T11:08:37+08:00

3
我没有看到任何JOIN aproaches，在重复方面有很多用途 .

这个aproeach给你实际的双倍结果 .
```
SELECT t1.* FROM table as t1 LEFT JOIN table as t2 ON t1.name=t2.name and t1.id!=t2.id WHERE t2.id IS NOT NULL ORDER BY t1.name
```
回复于 2024-04-29T11:08:37+08:00

SELECT 
    t.*,
    (SELECT COUNT(*) FROM city AS tt WHERE tt.name=t.name) AS count 
FROM `city` AS t 
WHERE 
    (SELECT count(*) FROM city AS tt WHERE tt.name=t.name) > 1 ORDER BY count DESC

回复于 2024-04-29T11:08:37+08:00

SELECT ColumnA, COUNT( * )
FROM Table
GROUP BY ColumnA
HAVING COUNT( * ) > 1

回复于 2024-04-29T11:08:37+08:00

1
我看到上面的结果，如果你需要检查重复的单列值，查询将正常工作 . 例如电子邮件 .

但是，如果您需要检查更多列，并希望检查结果的组合，以便此查询将正常工作：
```
SELECT COUNT(CONCAT(name,email)) AS tot,
       name,
       email
FROM users
GROUP BY CONCAT(name,email)
HAVING tot>1 (This query will SHOW the USER list which ARE greater THAN 1
              AND also COUNT)
```
回复于 2024-04-29T11:08:37+08:00
1286
以下将找到多次使用的所有product_id . 您只能为每个product_id获取一条记录 .
```
SELECT product_id FROM oc_product_reward GROUP BY product_id HAVING count( product_id ) >1
```
代码取自：http://chandreshrana.blogspot.in/2014/12/find-duplicate-records-based-on-any.html
回复于 2024-04-29T11:08:37+08:00

CREATE TABLE tbl_master
    (`id` int, `email` varchar(15));

INSERT INTO tbl_master
    (`id`, `email`) VALUES
    (1, 'test1@gmail.com'),
    (2, 'test2@gmail.com'),
    (3, 'test1@gmail.com'),
    (4, 'test2@gmail.com'),
    (5, 'test5@gmail.com');

QUERY : SELECT id, email FROM tbl_master
WHERE email IN (SELECT email FROM tbl_master GROUP BY email HAVING COUNT(id) > 1)

回复于 2024-04-29T11:08:37+08:00

12
一个非常晚的贡献......万一它可以帮助任何人下线...我有一个任务是在银行应用程序中找到匹配的交易对（实际上是账户到账户转账的两面），以确定哪些是每个账户间转账交易的“从”和“到”，所以我们最终得到了：
```
SELECT 
    LEAST(primaryid, secondaryid) AS transactionid1,
    GREATEST(primaryid, secondaryid) AS transactionid2
FROM (
    SELECT table1.transactionid AS primaryid, 
        table2.transactionid AS secondaryid
    FROM financial_transactions table1
    INNER JOIN financial_transactions table2 
    ON table1.accountid = table2.accountid
    AND table1.transactionid <> table2.transactionid 
    AND table1.transactiondate = table2.transactiondate
    AND table1.sourceref = table2.destinationref
    AND table1.amount = (0 - table2.amount)
) AS DuplicateResultsTable
GROUP BY transactionid1
ORDER BY transactionid1;
```
结果是 DuplicateResultsTable 提供了包含匹配（即重复）事务的行，但它也在第二次匹配同一对时反向提供相同的事务ID，因此外部 SELECT 是按第一个事务ID分组的，通过使用 LEAST 和 GREATEST 来确保两个transactionid在结果中始终处于相同的顺序，这使得第一个事件对于 GROUP 是安全的，从而消除了所有重复的匹配 . 超过近百万条记录，并在不到2秒的时间内确定了12,000场比赛 . 当然，transactionid是主要索引，这确实有帮助 .
回复于 2024-04-29T11:08:37+08:00
1
假设您的表名为TableABC，您想要的列是Col，而T1的主键是Key .
```
SELECT a.Key, b.Key, a.Col 
FROM TableABC a, TableABC b
WHERE a.Col = b.Col 
AND a.Key <> b.Key
```
这种方法优于上述答案的优势在于它给出了密钥 .
回复于 2024-04-29T11:08:37+08:00
1
根据levik的答案来获取重复行的ID，如果你的服务器支持它，你可以做 GROUP_CONCAT （这将返回逗号分隔的id列表） .
```
SELECT GROUP_CONCAT(id), name, COUNT(*) c FROM documents GROUP BY name HAVING c > 1;
```
回复于 2024-04-29T11:08:37+08:00

SELECT t.*,(select count(*) from city as tt
  where tt.name=t.name) as count
  FROM `city` as t
  where (
     select count(*) from city as tt
     where tt.name=t.name
  ) > 1 order by count desc

将 city 替换为您的表格 . 将 name 替换为您的字段名称

回复于 2024-04-29T11:08:37+08:00

SELECT varchar_col
FROM table
GROUP BY varchar_col
HAVING count(*) > 1;

回复于 2024-04-29T11:08:37+08:00

我更喜欢使用窗口函数（MySQL 8.0）来查找重复项，因为我可以看到整行：

WITH cte AS (
  SELECT *
    ,COUNT(*) OVER(PARTITION BY col_name) AS num_of_duplicates_group
    ,ROW_NUMBER() OVER(PARTITION BY col_name ORDER BY col_name2) AS pos_in_group
  FROM table
)
SELECT *
FROM cte
WHERE num_of_duplicates_group > 1;

DB Fiddle Demo

回复于 2024-04-29T11:08:37+08:00

SELECT DISTINCT a.email FROM `users` a LEFT JOIN `users` b ON a.email = b.email WHERE a.id != b.id;

回复于 2024-04-29T11:08:37+08:00

Select column_name, column_name1,column_name2, count(1) as temp from table_name group by column_name having temp > 1

回复于 2024-04-29T11:08:37+08:00

3
进一步使用@maxyfc's answer，我需要查找带有重复值返回的所有行，因此我可以在MySQL Workbench中编辑它们：
```
SELECT * FROM table
   WHERE field IN (
     SELECT field FROM table GROUP BY field HAVING count(*) > 1
   ) ORDER BY field
```
回复于 2024-04-29T11:08:37+08:00
2
使用 GROUP BY 子句执行 SELECT . 假设name是要查找重复项的列：
```
SELECT name, COUNT(*) c FROM table GROUP BY name HAVING c > 1;
```
这将返回第一列中名称值的结果，以及该值在第二列中出现的次数 .
回复于 2024-04-29T11:08:37+08:00
1
要查找Employee中名称列中有多少记录重复，下面的查询是有帮助的;
```
Select name from employee group by name having count(*)>1;
```
回复于 2024-04-29T11:08:37+08:00
144
我的最终查询包含了一些有用的答案 - 结合group by，count和GROUP_CONCAT .
```
SELECT GROUP_CONCAT(id), `magento_simple`, COUNT(*) c 
FROM product_variant 
GROUP BY `magento_simple` HAVING c > 1;
```
这提供了两个示例（逗号分隔）的ID，我需要的条形码以及重复的数量 .

相应地更改表格和列 .
回复于 2024-04-29T11:08:37+08:00

111

SELECT * 
FROM `dps` 
WHERE pid IN (SELECT pid FROM `dps` GROUP BY pid HAVING COUNT(pid)>1)

回复于 2024-04-29T11:08:37+08:00

186
要删除具有多个字段的重复行，首先将它们设置为为唯一不同的行指定的新唯一键，然后使用“group by”命令删除具有相同新唯一键的重复行：
```
Create TEMPORARY table tmp select concat(f1,f2) as cfs,t1.* from mytable as t1;
Create index x_tmp_cfs on tmp(cfs);
Create table unduptable select f1,f2,... from tmp group by cfs;
```
回复于 2024-04-29T11:08:37+08:00
9
```
SELECT  *
FROM    mytable mto
WHERE   EXISTS
        (
        SELECT  1
        FROM    mytable mti
        WHERE   mti.varchar_column = mto.varchar_column
        LIMIT 1, 1
        )
```
此查询返回完整记录，而不仅仅是不同的 varchar_column .

此查询不使用 COUNT(*) . 如果有很多重复项， COUNT(*) 是昂贵的，并且您不需要整个 COUNT(*) ，您只需要知道是否有两行具有相同的值 .

当然，在 varchar_column 上有一个索引会大大加快这个查询的速度 .
回复于 2024-04-29T11:08:37+08:00

在MySQL中查找重复值

22 回答

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