[] MySQL database data types

type of data

1. Integer

• Unsigned a certain number of non-negative

create table t3(
   age tinyint unsigned
);

• Display width (ZEROFILL)
  integral display width, when filled with enough digits 0

create table t4(
   id int(10) zerofill primary key auto_increment,
   name char(32)
);
insert into t4 values(12345, '5个');
insert into t4 values(1234567890, '10个');
insert into t4 values(123456789012, '12个');
select * from t4;

2. Float

Bits fixed point much longer use:
a float (M, D)
Double (M, D)
decimal (M, D)
M is the number of support length, D is the number of digits after the decimal point

create table t5 (
   a float(10, 2),
   b double(10, 2),
   c decimal(10, 2)
);

3. string type

The difference between CHAR and VARCHAR types

Think:
strings, so you can specify the size of the float freely, it's time to operate than usual casual finger
given a can it?
A: No, the data type is not the better, larger data type will result in a bloated, storage space is
occupied by large, data retrieval will be slow

4. Enumeration (enum)

When one kind selected from a plurality of types of data used
at the time of front-end radio button, enumerated type can function
enumerated type advantages: 1. Optional limit value
2. space saving
3. High operating efficiency

create table t6(
   name varchar(32),
   sex enum('男','女','保密') default '保密'
);
-- 枚举类型的计数默认从1开始
insert into t6 set name='王宝强',sex=1;

The collection (set)

SET can have a maximum of 64 different members. Similar to the check box, you can choose how many how many.

create table t7 (
   name varchar(32),
   hobby set('吃','睡','玩','喝','抽')
);
insert into t7 values('张三','睡,抽,玩,吃,喝');
insert into t7 values('李四','睡,抽');

• Why not use set type?
  In modern web development, multi-marquee value with thousands of stored value index with no more empty
• How to solve problems that box?
  The value of the check box of individually designed to a table

6. Time Type

1. datetime

create table datetime_test (
   create_at datetime
);
insert into datetime_test values('2019-4-2 16:54:00');
insert into datetime_test values('2019/4/2 16:54:00');
insert into datetime_test values(now());
-- 年份最大支持4个长度
insert into datetime_test values('10000/4/2 16:54:00');  --
错误
insert into datetime_test values('9999/4/2 16:54:00');

2.time

create table time_test (
   create_at time
);
insert into time_test values('12:12:12');
insert into time_test values('100:12:12');
insert into time_test values('-100:12:12');
insert into time_test values('10 10:12:12');
-- 时间的范围是: [-838:59:59 - 838:59:59]
insert into time_test values('839:12:12'); -- 错误的

3.timestamp timestamp type

Timestamp in the display area and datetime type is the same, is not the same in the memory
range from 1970-1-10: 2038-1-19 11:14:07 to 0: 0
indicates 4 bytes time stamp using
the value size storage for bit length: 2 * (4 * 8--1)

create table timestamp_test (
   create_time timestamp
);
insert into timestamp_test values(now());
insert into timestamp_test values('2038-1-19 11:14:07');  -
- 时间戳最大值
insert into timestamp_test values('2038-1-19 11:14:08');  -
- 错误

4.year

create table `year`(
   create_at year
);
-- 从1900年开始 - 1900+255
insert into `year` values(now());
insert into `year` values('2155'); -- 年份最大值
insert into `year` values('2156'); -- 错误

7. Boolean

create table `bool`(
   cond boolean
);
insert into `bool` set cond=True;    -- 成功
insert into `bool` set cond=False;   -- 成功
insert into `bool` set cond=1;       -- 成功
insert into `bool` set cond=10;      -- 成功
insert into `bool` set cond=-1;      -- 成功
insert into `bool` set cond=0;       -- 成功
insert into `bool` set cond=0.1;     -- 成功
insert into `bool` set cond='True';  -- 失败

8. The column properties

Whether the inserted value can be empty

• null: is empty, the default is not to write
• not null: not empty, if inserted, touch field is null, the error

create table null_test (
   id int primary key auto_increment,
   username varchar(32) not null,
   pwd varchar(16) null
);
insert into null_test values(null,null,null);

• default

create table default_test (
   id int primary key auto_increment,
   username varchar(32) default 'admin' not null,
   pwd varchar(16) default 123456
);
insert into default_test (username) values ('admin');

• auto_increment
  automatic growth column
  default starting from a
  normally used with the primary key

create table auto_inc (
   id int primary key auto_increment,
   name varchar(32)
);
insert into auto_inc (name) values ('aaa'), ('bbb'),
('ccc');
select * from auto_inc;
/* 输出：
+----+------+
| id | name |
+----+------+
| 1 | aaa |
| 2 | bbb |
| 3 | ccc |
+----+------+
*/

• primary key
  primary key uniquely identifies general
  characteristics: can not be empty, can not be repeated, a table which can have only one primary key

-- 这里只有一个主键,这种主键叫做联合主键, 在项目中使用较少
create table double_pri_test (
   id int,
   sid int,
   primary key(id,sid)
);
insert into double_pri_test values (1, 1);
insert into double_pri_test values (1, 2);  -- 成功
insert into double_pri_test values (2, 1);  -- 成功
insert into double_pri_test values (1, 1);  -- 失败

• unique
  unique key, to ensure that each data column which will not repeat the
  mailbox can not be repeated, the phone number can not be repeated

create table test_uniq (
   id int auto_increment primary key,
   mobile char(11) unique
);
insert into test_uniq set mobile=13999999999;

• comment
  field descriptions: the developer to see, for a general description of the respective fields

create table test_cmt (
   ctime datetime comment '这个字段代表创建日期'
);

9. SQL comments

Single-line comments: - Hello
multiline comment: / * barabara * /
MySQL unique single-line comments: ha ha ha ha #