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Mysql 字段类型与约束条件

时间:2020-01-03 12:31:41      阅读:103      评论:0      收藏:0      [点我收藏+]

标签:-128   字节   字段名   pytho   全局   哈哈   incr   字符串类型   特殊   

一、数值类型
二、日期类型
三、枚举与集合
四、约束条件
五、设置严格模式

一、数值类型

1.1 整型

应用场景: id号、年龄...

tinyint:
有符号:默认范围 -128, 127
无符号:默认范围 0,255

#tinyint 默认有符号:
create table t1(id tinyint,
                 name varchar(16)
                 );
insert into t1 values(-128,'tank'),(127,'jason');
insert into t1 values(-129,'sean') #报错
 
select * from t1;
+------+-------+
| id   | name  |
+------+-------+
| -128 | tank  |  #有符号,最小值为-128
|  127 | jason |  #有符号,最大值127
+------+-------+

#设置无符号tinyint
create table t2(id tinyint unsigned);

insert into t2 values((-1));  #报错
insert into t2 values((0)); 
insert into t2 values((255));
insert into t2 values((256));  #报错

select * from t2;
+------+
| id   |
+------+
|    0 |  #无符号,最小值为0
|  255 |  #无符号,最大值为255
+------+

int:
有符号:默认范围(-2147483648, 2147483647)
无符号:默认范围 (0,4294967295)
默认宽度为11

#int 默认有符号:
create table t3(id int);

insert into t3 values(-2147483648);
insert into t3 values(-2147483649); #报错
insert into t3 values(2147483647);
insert into t3 values(2147483648); #报错

select * from t3;
+-------------+
| id          |
+-------------+
| -2147483648 |  #有符号,最小值为-2147483648
|  2147483647 |  #有符号,最大值为2147483647
+-------------+  

#设置无符号int
create table t4(id int unsigned)

insert into t4 values((-1));
insert into t4 values((0));
insert into t4 values((4294967295));
insert into t4 values((4294967296));

select * from t4;
------------+
| id         |
+------------+
|          0 |  #无符号,最小值为0
| 4294967295 |  #无符号,最大值为4294967295
+------------+ 

1.2 浮点型

应用场景:存储薪资、身高、体重、体质参数等

float :FLOAT[(M,D)] ,单精度浮点数(非准确小数值)

double:DOUBLE[(M,D)],双精度浮点数(非准确小数值)

以上两个的m是数字总个数,d是小数点后个数。m最大值为255,d最大值为30

decimal:DECIMAL[(M,D)],准确的小数值
m最大值为65,d最大值为30。

# 范围255是最大长度(包括.小数), 30代表是小数的位数
create table t5(x float(255, 30));
create table t6(x double(255, 30));
create table t7(x decimal(65, 30));

# 三种浮点型: 区别在于精确度不一样
insert into t5 values(1.111111111111111111111111111111);
insert into t6 values(1.1111111111111111111111111111);
insert into t7 values(1.1111111111111111111111111111);

#随着小数的增多,精度开始不准确
mysql> select * from t5;  
+----------------------------------+
| x                                |
+----------------------------------+
| 1.111111164093017600000000000000 |

#精度比float要准确点,但随着小数的增多,同样变得不准确
mysql> select * from t6;
+----------------------------------+
| x                                |
+----------------------------------+
| 1.111111111111111200000000000000 |

#精度始终准确,d为30,于是只留了30位小数
mysql> select * from t7;
+----------------------------------+
| x                                |
+----------------------------------+
| 1.111111111111111111111111111100 |
+----------------------------------+

技术图片

1.4 字符串类型

char: 定长,简单粗暴,浪费空间,存取速度快。

字符长度范围:0-255(一个中文是一个字符,是utf8编码的3个字节)。

存储char类型的值时,会往右填充空格来满足长度

create table t8(id int,name char(4));

insert into t8 values(1, 'tank');
insert into t8 values(1, 't');  
insert into t8 values(1, '你个大傻子'); #字符长度超过4报错

mysql> select * from t8;
+------+--------------+
| id   | name         |
+------+--------------+
|    1 | tank         |
|    1 | t            |   # t+3个空格
+------+--------------+

varchar: 不定长字符,精准,节省空间,存取速度慢

存几个字符,就是几个字符的大小,每个字符前都要+1bytes

create table t9(id int, name varchar(4));

insert into t9 values(1, 'bob');  # 1bytes + bob
insert into t9 values(2, 'tank');  # 1bytes + tank
insert into t9 values(3, 'sean');  # 1bytes + sean
insert into t9 values(4, 'jason'); #报错

mysql> select * from t9;
+------+------+
| id   | name |
+------+------+
|    1 | bob  |
|    2 | tank |
|    3 | sean |
+------+------+

技术图片

二、日期类型

技术图片
例如:
date: 2019-12-11
datetime: 2019-12-11 11:11:11
time: 11:11:11
year: 2019
timestamp: 时间戳

注意:TIMESTAMP的字段默认不为空(not null),默认值为当前时间(CURRENT_TIMESTAMP),如果不做特殊处理,并且update语句中没有指定该列的更新值,则默认更新为当前时间。

create table student(
    id int,
    name varchar(10),
    birth date,
    register datetime,
    born_year year,
    t_time time,
    update_time timestamp
    );
          
insert into student values(1, 'baohan', '1996-06-23', '2019-12-12 11:11:11', '1996', '11:11:11', null);
insert into student values(2, 'HCY', '1000-11-11', '1980-11-11 11:11:11','2019', '11:11:11', null);

mysql> select * from student;
+------+--------+------------+---------------------+-----------+----------+---------------------+
| id   | name   | birth      | register            | born_year | t_time   | update_time         |
+------+--------+------------+---------------------+-----------+----------+---------------------+
|    1 | baohan | 1996-06-23 | 2019-12-12 11:11:11 |      1996 | 11:11:11 | 2019-12-11 16:02:45 |
|    2 | HCY    | 1000-11-11 | 1980-11-11 11:11:11 |      2019 | 11:11:11 | 2019-12-11 16:03:22 |
+------+--------+------------+---------------------+-----------+----------+---------------------+

三、枚举与集合

字段的值只能在给定范围中选择,如单选框,多选框
enum: 单选 只能在给定的范围内选一个值(多选一),如性别 sex 男male/女female
set : 多选 在给定的范围内可以选择一个或一个以上的值 ,如(爱好1,爱好2,爱好3...)

#enum:多选一
create table t10(id int, name varchar(4), gender enum('male', 'female', 'others'));

# insert into 表名(字段名) values(字段名对应的值);
insert into t13(id, name, gender) values(1, 'tank', 'male');
insert into t13(id, name, gender) values(1, 'tank', '人妖'); #报错,‘人妖’不在范围内

mysql> select * from t10;
+------+------+--------+
| id   | name | gender |
+------+------+--------+
|    1 | tank | male   |
+------+------+--------+
#set 可 多选一 或 多选多
create table t11(
    id int,
    name varchar(4),
    hobbies set('read', 'sing', 'run',  'think')
    );

insert into t11 values(1, 'bob', 'read,sing');

mysql> select * from t11;
+------+------+-----------+
| id   | name | hobbies   |
+------+------+-----------+
|    1 | bob  | read,sing |
+------+------+-----------+

四、约束条件

初始约束条件:not null

#约束插入记录时id不能为空
create table t1(id int not null,name varchar(6));

insert into t1 values(1,'tank');
insert into t1 values(null,'tank');  #报错

mysql> select * from t1;
+----+------+
| id | name |
+----+------+
|  1 | tank |
+----+------+

unique: :唯一,将某个字段设置为唯一的值

create table t2(id int not null unique,name varchar(4));

insert into t2(id, name) values(1, 'tank'), (2, 'sean');
insert into t2(id, name) values(1, 'bob'); #报错,1已用过

+----+------+
| id | name |
+----+------+
|  1 | tank |
|  2 | sean |
+----+------+

primary key(主键)----> not null unique: pk就是表中的索引: 可以通过索引快速查找某些数据,提高查询效率。

# 将id设置为主键,非空且唯一
create table t3(id int primary key,name varchar(4));

insert into t3(id, name) values(1, 'tank');
insert into t3(id, name) values(2, 'sean');
insert into t3(id, name) values(1, 'bob'); #报错 

mysql> desc t3;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id    | int(11)    | NO   | PRI | NULL    |       |
| name  | varchar(4) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+

auto_increment:自增

# 将id设置为自增
create table t4(id int primary key auto_increment,name varchar(4));

# 自增默认从0开始
insert into t4(name) values('tank');
insert into t4(name) values('sean');
insert into t4(name) values('bob');
insert into t4(name) values('哈哈哥');

mysql> select * from t4;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | tank      |
|  2 | sean      |
|  3 | bob       |
|  4 | 哈哈哥    |
+----+-----------+

## 若想自增从指定值开始,可插入第一条数据时先指定id的值;
insert into t4(id, name) values(10, 'tank');
insert into t4(name) values('sean');   # 11
insert into t4(name) values('bob');   # 12
insert into t4(name) values('哈哈哥');  # 13

mysql> select * from t4;
+----+-----------+
| id | name      |
+----+-----------+
| 10 | tank      |
| 11 | sean      |
| 12 | bob       |
| 13 | 哈哈哥    |
+----+-----------+

zerofill:使用0填充空格

create table t5(id int zerofill);

insert into t5 values(100);

mysql> select * from t5;
+------------+
| id         |
+------------+
| 0000000100 |
+------------+

default:约束插入数据时的默认值

create table t7(id int,
                name varchar(32) unique,
                password varchar(255) default '000000');    

insert into t7(id,name) values(1,'tank');

mysql> select * from t7;
+------+------+----------+
| id   | name | password |
+------+------+----------+
|    1 | tank | 000000   |
+------+------+----------+

delete:清空表中所有记录:
truncate:清空表中的所有记录,并且id重置为0

#delete
create table t6(id int primary key auto_increment,name varchar(4));

insert into t6(name) values('tank');
insert into t6(name) values('大大大'), ('sean');
delete from t6;
insert into t6(name) values('tank');
insert into t6(name) values('大大大'), ('sean');
    
mysql> select * from t6;
+----+-----------+
| id | name      |
+----+-----------+
|  4 | tank      |
|  5 | 大大大    |
|  6 | sean      |
+----+-----------+



#truncate
create table t6(id int primary key auto_increment,name varchar(4));

insert into t6(name) values('tank');
insert into t6(name) values('sean');
truncate table t6;
insert into t6(name) values('tank');
insert into t6(name) values('sean');

mysql> select * from t6;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | tank      |
|  2 | sean      |
+----+-----------+

五、设置严格模式

# 查看数据库配置中变量名包含mode的配置参数:
show variables like "%mode%";

# 修改安全模式:
set session;  # 局部有效,只在你当前操作的窗口有效
set global session;  # 全局有效,永久有效

# 修改完之后退出当前客户端重新登录即可,不用关闭服务端
set global sql_mode = 'STRICT_TRANS_TABLES';

Mysql 字段类型与约束条件

标签:-128   字节   字段名   pytho   全局   哈哈   incr   字符串类型   特殊   

原文地址:https://www.cnblogs.com/baohanblog/p/12143996.html

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