MySQL处理重复数据实例分析

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MySQL 限制数据重复的方式：表上增加主键（Primary Key）或增加唯一性索引（Unique）

主键对重复资料进行限制，这样资料在导入时就无法重复插入

create table primary_t(last_name varchar(20),
first_name varchar(20),
sex varchar(8),
primary key(last_name,first_name)
)engine=innodb default charset=utf8;



show table status like 'primary_t'\G

show index from primary_t\G


模拟插入两笔有主键约束的资料
insert into primary_t(last_name,first_name,sex) values('Frank','Li','Man'),('Frank','Li','Man'),('Coco','Li','Woman');

ERROR 1062 (23000): Duplicate entry 'Frank-Li' for key 'PRIMARY'


insert into primary_t(last_name,first_name,sex) values('Frank','Li','Man'),('Frank','Cai','Man'),('Coco','Li','Woman');

mysql> select * from primary_t;
+-----------+------------+-------+
| last_name | first_name | sex   |
+-----------+------------+-------+
| Coco      | Li         | Woman |
| Frank     | Cai        | Man   |
| Frank     | Li         | Man   |
+-----------+------------+-------+
rows in set (0.00 sec)

唯一性索引对复制资料进行限制

create table unique_t(last_name varchar(20),
first_name varchar(20),
sex varchar(8),
unique index  unique_name(last_name,first_name)
)engine=innodb default charset=utf8;

show table status like 'unique_t'\G

show index from unique_t\G


模拟插入两笔有唯一性约束的资料
insert into unique_t(last_name,first_name,sex) values('Frank','Li','Man'),('Frank','Li','Man'),('Coco','Li','Woman');

ERROR 1062 (23000): Duplicate entry 'Frank-Li' for key 'unique_name'


insert into unique_t(last_name,first_name,sex) values('Frank','Li','Man'),('Frank','Cai','Man'),('Coco','Li','Woman');
mysql> select * from unique_t;
+-----------+------------+-------+
| last_name | first_name | sex   |
+-----------+------------+-------+
| Frank     | Li         | Man   |
| Frank     | Cai        | Man   |
| Coco      | Li         | Woman |
+-----------+------------+-------+
rows in set (0.00 sec)

对资料的统计

create table count_t(last_name varchar(20),
first_name varchar(20),
sex varchar(8)
)engine=innodb default charset=utf8;

insert into count_t values('Frank','A','Man'),
('Frank','B','Man'),
('Frank','C','Woman'),
('Frank','D','Man'),
('Frank','E','Man'),
('Frank','F','Woman'),
('Frank','G','Man'),
('Frank','H','Man'),
('Frank','I','Woman'),
('Coco','A','Woman'),
('Coco','B','Man'),
('Coco','C','Man'),
('Coco','D','Man'),
('Coco','E','Man'),
('Coco','F','Man'),
('Coco','G','Woman'),
('Coco','H','Woman'),
('Alex','B','Man'),
('ALex','C','Man'),
('ALex','D','Man'),
('Alex','E','Man'),
('ALex','F','Man'),
('ALex','G','Woman'),
('ALex','H','Woman')
;

commit;

按各个列进行分组统计
select last_name,count(*) from count_t group by last_name;


select first_name,count(*) from count_t group by first_name;


select sex,count(*) from count_t group by sex;


mysql> select last_name,count(*) from count_t group by last_name;
+-----------+----------+
| last_name | count(*) |
+-----------+----------+
| Alex      |        7 |
| Coco      |        8 |
| Frank     |        9 |
+-----------+----------+
rows in set (0.00 sec)


mysql> select first_name,count(*) from count_t group by first_name;
+------------+----------+
| first_name | count(*) |
+------------+----------+
| A          |        2 |
| B          |        3 |
| C          |        3 |
| D          |        3 |
| E          |        3 |
| F          |        3 |
| G          |        3 |
| H          |        3 |
| I          |        1 |
+------------+----------+
rows in set (0.00 sec)


mysql> select sex,count(*) from count_t group by sex;
+-------+----------+
| sex   | count(*) |
+-------+----------+
| Man   |       16 |
| Woman |        8 |
+-------+----------+
rows in set (0.00 sec)


对分组的统计在进行限定，having 大于等于 8 次的记录
select last_name ,count(*) from count_t group by last_name having count(*) >=8;

mysql> select last_name ,count(*) from count_t group by last_name having count(*) >=8;
+-----------+----------+
| last_name | count(*) |
+-----------+----------+
| Coco      |        8 |
| Frank     |        9 |
+-----------+----------+
rows in set (0.00 sec)

对资料进行去重操作，主要使用 distinct 函数

select distinct last_name from count_t;
select distinct first_name from count_t;
select distinct sex from count_t;

对已有资料的表进行去重操作

insert into count_t values('Xi','Xi','Man'),('Xi','Xi','Man');


增加主键约束，加入 ignore，忽悠重复的资料

alter ignore table count_t
add primary key(last_name,first_name);

select * from count_t where last_name='Xi';



mysql> alter ignore table count_t
    -> add primary key(last_name,first_name);
Query OK, 26 rows affected (0.01 sec)
Records: 26  Duplicates: 1  Warnings: 0

mysql> select * from count_t where last_name='Xi';
+-----------+------------+------+
| last_name | first_name | sex  |
+-----------+------------+------+
| Xi        | Xi         | Man  |
+-----------+------------+------+
row in set (0.00 sec)


使用 group by 方式也可去除重复资料

create table count_t0 as select last_name,first_name,sex from count_t group by last_name,first_name,sex;

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