我的实例 代理统计表删除资讯重复记录
-- select * from daili_workcount a
-- where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1)
-- create table `tmptable` as (
-- select id,type,work_id from daili_workcount a
-- where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1));
DELETE from daili_workcount WHERE id in(
select b.id from
(
select a.id from daili_workcount a
where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1)
and a.id not in (select min(id) from daili_workcount group by type,work_id having count(*)>1)
) b
)来源文章：
[教学] MySQL查询重复字段,及删除重复记录的方法
MySQL, 数据库, 数据库, 字段, 服务器
数据库中有个大表，需要查找其中的名字有重复的记录id，以便比较。如果仅仅是查找数据库中name不重复的字段，很容易:
SELECT min(id),name FROM table GROUP BY name;
但是这样并不能得到说有重复字段的id值。（只得到了最小的一个id值）查询哪些字段是重复的也容易:
SELECT name,count(name) as count FROM table GROUP BY name HAVING count(name) >1 ORDER BY count DESC;
但是要一次查询到重复字段的id值，就必须使用子查询了，于是使用下面的语句。
SELECT id,name FROM table WHERE name in (
SELECT name
FROM table
GROUP BY name HAVING count(name) >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零时表。于是使用先建立零时表:
create table tmptable as (
SELECT name
FROM table
GROUP BY name HAVING count(name) >1);
然后使用多表连接查询：
SELECT a.id, a.name FROM table a, tmptable t WHERE a.name = t.name;
结果这次结果很快就出来了。
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查询及删除重复记录的方法
(一)
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录
delete from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1)
3、查找表中多余的重复记录（多个字段）
select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
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、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
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)
比方说
在A表中存在一个字段「name」，
而且不同记录之间的「name」值有可能会相同，
现在就是需要查询出在该表中的各记录之间，「name」值存在重复的项；
Select Name,Count() From A Group By Name Having Count() > 1
如果还查性别也相同大则如下:
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
方法二
有两个意义上的重复记录，一是完全重复的记录，也即所有字段均重复的记录，二是部分关键词段重复的记录，比如Name字段重复，而其它字段不一定重复或都重复可以忽略。
1、对于第一种重复，比较容易解决，使用
select distinct * from tableName
就可以得到无重复记录的结果集。
如果该表需要删除重复的记录（重复记录保留1条），可以按以下方法删除
select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp
发生这种重复的原因是表设计不周产生的，增加唯一索引列即可解决。
2、这类重复问题通常要求保留重复记录中的第一条记录，操作方法如下
假设有重复的字段为Name,Address，要求得到这两个字段唯一的结果集
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)
最后一个select即得到了Name，Address不重复的结果集（但多了一个autoID字段，实际写时可以写在select子句中省去此列）
(四)
查询重复
select * from tablename where id in (
select id from tablename
group by id
having count(id) > 1
)
mysql中You can’t specify target table for update in FROM clause错误解决方法
mysql中You can’t specify target table for update in FROM clause错误的意思是说，不能先select出同一表中的某些值，再update这个表(在同一语句中)。 例如下面这个sql：
delete from tbl where id in
(
select max(id) from tbl a where EXISTS
(
select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1
)
group by tac
)
delete from tbl where id in
(
select a.id from
(
select max(id) id from tbl a where EXISTS
(
select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1
)
group by tac
) a
)也就是说将select出的结果再通过中间表select一遍，这样就规避了错误。注意，这个问题只出现于mysql，mssql和oracle不会出现此问题。