MySQL的重複數據刪除

Question

這顯示了我所有的名字，並且恰好具有兩個條目是相同

SELECT `firstname`,`lastname`,COUNT(*) AS Count 
FROM `people` 
GROUP BY `firstname`,`lastname` 
HAVING Count = 2 

姓氏如何變成這個DELETE FROM WHERE用了限制語句只刪除一個的每個條目，並離開另一個。

還好這似乎是辦法的技術我只是打算做一個PHP while循環

Answer 1

您可以創建一個包含每個副本1條記錄的表：然後刪除people表中的所有dup記錄，然後重新插入dup記錄。

-- Setup for example 
create table people (fname varchar(10), lname varchar(10)); 

insert into people values ('Bob', 'Newhart'); 
insert into people values ('Bob', 'Newhart'); 
insert into people values ('Bill', 'Cosby'); 
insert into people values ('Jim', 'Gaffigan'); 
insert into people values ('Jim', 'Gaffigan'); 
insert into people values ('Adam', 'Sandler'); 

-- Show table with duplicates 
select * from people; 

-- Create table with one version of each duplicate record 
create table dups as 
    select distinct fname, lname, count(*) 
    from people group by fname, lname 
    having count(*) > 1; 

-- Delete all matching duplicate records 
delete people from people inner join dups 
on people.fname = dups.fname AND 
    people.lname = dups.lname; 

-- Insert single record of each dup back into table 
insert into people select fname, lname from dups; 

-- Show Fixed table 
select * from people; 

Answer 2

，如果你有一個主鍵，如ID，你可以這樣做：

delete from people 
where id not in 
(
     select minid from 
     (select min(id) as minid from people 
     group by firstname, lastname) as newtable 
) 

子查詢select min(id)...位爲您提供給定名字，姓氏組合的唯一（基於id）行;然後你刪除所有其他行，即你的重複。你需要你的子查詢包裹由於在MySQL中的錯誤，否則我們可以這樣做：

delete from people 
where id not in 
(
     select min(id) as minid from people 
     group by firstname, lastname 
) 

更好的方式是：

delete people from 
people left outer join 
(
    select min(id) as minid from people 
    group by firstname, lastname 
) people_grouped 
on people.first_name = people_grouped.first_name 
and people.last_name = people_grouped.last_name 
and people_grouped.id is null 

避免子查詢。

Answer 3

創建一個新表並在（名，姓）上添加一個唯一鍵。然後將舊錶中的行插入新表中。然後重命名錶格。

mysql> select * from t; 
+-----------+----------+ 
| firstname | lastname | 
+-----------+----------+ 
| A   | B  | 
| A   | B  | 
| X   | Y  | 
+-----------+----------+ 
3 rows in set (0.00 sec) 

mysql> create table t2 like t; 
Query OK, 0 rows affected (0.00 sec) 

mysql> alter table t2 add unique key name(firstname,lastname); 
Query OK, 0 rows affected (0.00 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> insert ignore into t2 select * from t; 
Query OK, 2 rows affected (0.00 sec) 
Records: 3 Duplicates: 1 Warnings: 0 


mysql> select * from t2; 
+-----------+----------+ 
| firstname | lastname | 
+-----------+----------+ 
| A   | B  | 
| X   | Y  | 
+-----------+----------+ 
2 rows in set (0.01 sec)