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
代码语言：javascript复制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_tG
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
代码语言：javascript复制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_tG
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
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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;
