INSERT INTO hk_test(username, passwd) VALUES
('qmf1', 'qmf1'),('qmf2', 'qmf11')
delete from hk_test where username='qmf1' and passwd='qmf1'
MySQL里查詢表里的重復數據記錄：
先查看重復的原始數據：
場景一：列出username字段有重讀的數據
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;
這種方法只是統計了該字段重復對應的具體的個數
場景二：列出username字段重復記錄的具體指：
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)
但是這條語句在mysql中效率太差，感覺mysql并沒有為子查詢生成臨時表。在數據量大的時候，耗時很長時間
解決方法：
于是使用先建立臨時表
create table `tmptable` as (
SELECT `name`
FROM `table`
GROUP BY `name` HAVING count(`name`) >1
);
然后使用多表連接查詢
SELECT a.`id`, a.`name`
FROM `table` a, `tmptable` t
WHERE a.`name` = t.`name`;
結果這次結果很快就出來了。
用 distinct去重復
SELECT distinct a.`id`, a.`name`
FROM `table` a, `tmptable` t
WHERE a.`name` = t.`name`;
場景三：查看兩個字段都重復的記錄：比如username和passwd兩個字段都有重復的記錄：
select * from hk_test a
where (a.username,a.passwd) in (select username,passwd from hk_test group by username,passwd having count(*) > 1)
場景四：查詢表中多個字段同時重復的記錄：
select username,passwd,count(*) from hk_test group by username,passwd having count(*) > 1
MySQL查詢表內重復記錄
查詢及刪除重復記錄的方法
(一)
1、查找表中多余的重復記錄，重復記錄是根據單個字段（peopleId）來判斷
select *
from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId)>1)
2、刪除表中多余的重復記錄，重復記錄是根據單個字段（peopleId）來判斷，只留有一個記錄
delete from people
where peopleId in (select peopleId
from people group by peopleId having count(peopleId)>1)
and min(id) not
in (select id from people group by peopleId having count(peopleId)>1)
3、查找表中多余的重復記錄（多個字段）
select * from vitae a
where (a.peopleId,a.seq) in
(select peopleId,seq from vitae group by peopleId,seq having count(*)>1)
4、刪除表中多余的重復記錄（多個字段），只留有rowid最小的記錄
delete from vitae a
where
(a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq
having count(*) > 1)
and rowid not in (select min(rowid) from vitae group
by peopleId,seq having count(*)>1)
5、查找表中多余的重復記錄（多個字段），不包含rowid最小的記錄
select * from vitae a
where
(a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq
having count(*) > 1)
and rowid not in (select min(rowid) from vitae group
by peopleId,seq having count(*)>1)
(二)
比方說
在A表中存在一個字段“name”，而且不同記錄之間的“name”值有可能會相同，現在就是需要查詢出在該表中的各記錄之間，“name”值存在重復的項；
Select Name,Count(*) From A Group By Name Having Count(*) > 1
如果還查性別也相同大則如下:
Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
(三)
方法一
declare @max integer,@id integer
declare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段
having count(*) >； 1
open cur_rows
fetch cur_rows into @id,@max
while @@fetch_status=0
begin
select @max = @max -1
set rowcount @max
delete from 表名 where 主字段 = @id
fetch cur_rows into @id,@max
end
close cur_rows
set rowcount 0
SELECT * from tab1 where CompanyName in( SELECT companyname from tab1 GROUP BY CompanyName HAVING COUNT(*)>1);
-- 129.433ms
SELECT * from tab1 INNER join ( SELECT companyname from tab1 GROUP BY CompanyName HAVING COUNT(*)>1) as tab2 USING(CompanyName);
-- 0.482ms
方法二
有兩個意義上的重復記錄，一是完全重復的記錄，也即所有字段均重復的記錄，二是部分關鍵字段重復的記錄，比如Name字段重復，而其他字段不一定重復或都重復可以忽略。
1、對于第一種重復，比較容易解決，使用
select distinct * from tableName
就可以得到無重復記錄的結果集。
如果該表需要刪除重復的記錄（重復記錄保留1條），可以按以下方法刪除
select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp
發生這種重復的原因是表設計不周產生的，增加唯一索引列即可解決。
2、這類重復問題通常要求保留重復記錄中的第一條記錄，操作方法如下
假設有重復的字段為Name,Address，要求得到這兩個字段唯一的結果集
select identity(int,1,1) as autoID, * into #Tmp from tableName
select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,autoID
select * from #Tmp where autoID in(select autoID from #tmp2)
最后一個select即得到了Name，Address不重復的結果集（但多了一個autoID字段，實際寫時可以寫在select子句中省去此列）
(四)查詢重復
select * from tablename where id in (
select id from tablename group by id having count(id) > 1)
常用的語句
1、查找表中多余的重復記錄，重復記錄是根據單個字段（mail_id）來判斷
代碼如下 復制代碼
SELECT * FROM table WHERE mail_id IN (SELECT mail_id FROM table GROUP BY mail_id HAVING COUNT(mail_id) > 1);
2、刪除表中多余的重復記錄，重復記錄是根據單個字段（mail_id）來判斷，只留有rowid最小的記錄
代碼如下 復制代碼
DELETE FROM table WHERE mail_id IN (SELECT mail_id FROM table GROUP BY mail_id HAVING COUNT(mail_id) > 1) AND rowid NOT IN (SELECT MIN(rowid) FROM table GROUP BY mail_id HAVING COUNT(mail_id )>1);
3、查找表中多余的重復記錄（多個字段）
代碼如下 復制代碼
SELECT * FROM table WHERE (mail_id,phone) IN (SELECT mail_id,phone FROM table GROUP BY mail_id,phone HAVING COUNT(*) > 1);
4、刪除表中多余的重復記錄（多個字段），只留有rowid最小的記錄
代碼如下 復制代碼
DELETE FROM table WHERE (mail_id,phone) IN (SELECT mail_id,phone FROM table GROUP BY mail_id,phone HAVING COU(www.jb51.net)NT(*) > 1) AND rowid NOT IN (SELECT MIN(rowid) FROM table GROUP BY mail_id,phone HAVING COUNT(*)>1);
5、查找表中多余的重復記錄（多個字段），不包含rowid最小的記錄
代碼如下 復制代碼
SELECT * FROM table WHERE (a.mail_id,a.phone) IN (SELECT mail_id,phone FROM table GROUP BY mail_id,phone HAVING COUNT(*) > 1) AND rowid NOT IN (SELECT MIN(rowid) FROM table GROUP BY mail_id,phone HAVING COUNT(*)>1);
存儲過程
declare @max integer,@id integer
declare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段 having count(*) >； 1
open cur_rows
fetch cur_rows into @id,@max
while @@fetch_status=0
begin
select @max = @max -1
set rowcount @max
delete from 表名 where 主字段 = @id
fetch cur_rows into @id,@max
end
close cur_rows
set rowcount 0
(一)單個字段
1、查找表中多余的重復記錄，根據（question_title）字段來判斷
代碼如下 復制代碼
select * from questions where question_title in (select question_title from people group by question_title having count(question_title) > 1)
2、刪除表中多余的重復記錄，根據（question_title）字段來判斷，只留有一個記錄
代碼如下 復制代碼
delete from questions
where peopleId in (select peopleId from people group by peopleId having count(question_title) > 1)
and min(id) not in (select question_id from questions group by question_title having count(question_title)>1)
(二)多個字段
刪除表中多余的重復記錄（多個字段），只留有rowid最小的記錄
代碼如下 復制代碼
DELETE FROM questions WHERE (questions_title,questions_scope) IN (SELECT questions_title,questions_scope FROM que(www.jb51.net)stions GROUP BY questions_title,questions_scope HAVING COUNT(*) > 1) AND question_id NOT IN (SELECT MIN(question_id) FROM questions GROUP BY questions_scope,questions_title HAVING COUNT(*)>1)
用上述語句無法刪除，創建了臨時表才刪的，求各位達人解釋一下。
代碼如下 復制代碼
CREATE TABLE tmp AS SELECT question_id FROM questions WHERE (questions_title,questions_scope) IN (SELECT questions_title,questions_scope FROM questions GROUP BY questions_title,questions_scope HAVING COUNT(*) > 1) AND question_id NOT IN (SELECT MIN(question_id) FROM questions GROUP BY questions_scope,questions_title HAVING COUNT(*)>1);
DELETE FROM questions WHERE question_id IN (SELECT question_id FROM tmp);
DROP TABLE tmp;
查找mysql數據表中重復記錄
mysql數據庫中的數據越來越多,當然排除不了重復的數據,在維護數據的時候突然想到要把多余的數據給刪減掉,剩下有價值的數據。
以下sql語句可以實現查找出一個表中的所有重復的記錄.
select user_name,count(*) as count from user_table group by user_name having count>1;
參數說明:
user_name為要查找的重復字段.
count用來判斷大于一的才是重復的.
user_table為要查找的表名.
group by用來分組
having用來過濾.
把參數換成自己數據表的相應字段參數，可以先在Phpmyadmin里面或者Navicat里面去運行，看看有哪些數據重復了，然后在數據庫里面刪除掉，也可以直接將SQL語句放到后臺讀取新聞的頁面里面讀取出來，完善成查詢重復數據的列表，有重復的可以直接刪除。
效果如下：
缺點：這種方法的缺點就是當你的數據庫里面的數據量很大的時候，效率很低，我用的是Navicat測試的，數據量不大，效率很高，當然，網站還有其它查詢數據重復的SQL語句，舉一反三，大家好好研究研究，找到一個適合自己網站的查詢語句。
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