mysql子查询性能,mysql查询表里的重复数据方法 – 数据库 – 前端，mysql order by null

mysql复合索引，mysql update a b，RedHat忘记MySQL密码，mysql utf8 表情，mysql清空datatable，mysql order by null先查看重复的原始数据：

场景一：列出username字段有重读的数据

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select username,count(*) as count from hk_test group by username having count>1;

SELECT username,count(username) as count FROM hk_test GROUP BY username HAVING count(username) >1 ORDER BY count DESC;

这种方法只是统计了该字段重复对应的具体的个数

场景二：列出username字段重复记录的具体指：

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select * from hk_test where username in (select username from hk_test group by username having count(username) > 1)

SELECT username,passwd FROM hk_test WHERE username in ( SELECT username FROM hk_test GROUP BY username HAVING count(username)>1) 但是这条语句在mysql中效率太差，感觉mysql并没有为子查询生成临时表。在数据量大的时候，耗时很长时间

解决方法：

场景三：查看两个字段都重复的记录：比如username和passwd两个字段都有重复的记录：

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select * from hk_test a

where (a.username,a.passwd) in (select username,passwd from hk_test group by username,passwd having count(*) > 1)

场景四：查询表中多个字段同时重复的记录：

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select username,passwd,count(*) from hk_test group by username,passwd having count(*) > 1

参数说明:

user_name为要查找的重复字段.

count用来判断大于一的才是重复的.

user_table为要查找的表名.

group by用来分组

having用来过滤.

mysql复合索引，mysql update a b，RedHat忘记MySQL密码，mysql utf8 表情，mysql清空datatable，mysql order by null