如果要删除具有多个字段的重复行，首先将它们取消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;

要查找Employee中的name列中有多少记录是重复的，下面的查询很有用;

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

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

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

我从下面开始改进:

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

如果要删除具有多个字段的重复行，首先将它们取消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;