在MySQL中查找重复值

Question

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？

Answer 1

做SELECT一个GROUP BY条款.假设name是要在以下位置找到重复项的列:

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.

Answer 2

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

Answer 3

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varchar_column的记录.

此查询不使用COUNT(*).如果有很多重复,COUNT(*)很昂贵,而且你不需要整体COUNT(*),你只需要知道是否有两行具有相同的值.

varchar_column当然,拥有一个索引会大大加快这个查询的速度.

Answer 4

GROUP_CONCAT如果你的服务器支持,你可以根据levik的答案来获取重复行的ID,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;

Answer 5

为了获取包含重复的所有数据，我使用了这个：

SELECT * FROM TableName INNER JOIN(
  SELECT DupliactedData FROM TableName GROUP BY DupliactedData HAVING COUNT(DupliactedData) > 1 order by DupliactedData)
  temp ON TableName.DupliactedData = temp.DupliactedData;

TableName = 您正在使用的表。

DupliactedData = 您要查找的重复数据。

Answer 6

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

Answer 7

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.

Answer 8

要查找Employee中名称列中有多少记录重复,下面的查询很有用;

Select name from employee group by name having count(*)>1;

Answer 9

我没有看到任何JOIN aproaches,在重复方面有很多用途.

这个aproeach为您提供实际的双倍结果.

SELECT t1.* FROM my_table as t1 
LEFT JOIN my_table as t2 
ON t1.name=t2.name and t1.id!=t2.id 
WHERE t2.id IS NOT NULL 
ORDER BY t1.name

Answer 10

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

用您的表替换城市.将名称替换为您的字段名称

Answer 11

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.

Answer 12

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,那么此查询将正常工作:

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)

Answer 13

以@ maxyfc的答案进一步，我需要找到所有与重复的值返回的行，这样我就可以在编辑MySQL工作台：

SELECT * FROM table
   WHERE field IN (
     SELECT field FROM table GROUP BY field HAVING count(*) > 1
   ) ORDER BY field

Answer 14

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

Answer 15

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;

数据库小提琴演示

Answer 16

我从此改进：

SELECT 
    col, 
    COUNT(col)
FROM
    table_name
GROUP BY col
HAVING COUNT(col) > 1;