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查询   columnName 有重复的数据，保留  id  最大的一个。

SELECT MAX(id) AS id, columnName, COUNT(columnName  ) AS countNumber
FROM tableName
GROUP BY columnName
HAVING countNumber> 1
ORDER BY countNumber DESC;


删除  columnName  有重复的数据，保留  id  最小的一个。

DELETE tableName t1
FROM tableName t1, (SELECT min(id) id , columnName
 FROM tableName
 GROUP BY columnName
 HAVING COUNT(1) > 1
 ) t2
WHERE t1.columnName = t2.columnName
AND t1.   id > t2.id;