Basic data types of MySQL

What is the data type ？ To put it bluntly, it is the storage format of data 、 Constraints and effective range .

Selection of data type , That is to specify the underlying data storage structure of different data , Directly affect the performance of the database .

The selection of data type is also an important point of database performance tuning , Learning data types carefully is also a preparation for learning database tuning in the future .

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How do I know which data types there are in the database ？

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Check the data types

Specific syntax and instructions for each data type

You can see the data type ：int Specific syntax and instructions （ Mainly the effective range ）

All the data types listed here can be used to view the specific syntax and description in this way , Therefore, there is no need to deliberately remember the specific details of these data types , Remember and forget , Learning how to check is more meaningful than this .

Next, I will explain several commonly used data types .

1. value type

1 Bytes =8 Bit binary ,8 position =2^8=256, therefore 1 Total length of bytes =256

1111 1111=255, therefore 1 The maximum stored value of bytes is 255

among “0000 0000”=0 It's also a value ,1+255=256=2^8

So you know 1 Total length of bytes and Storing maximum values is not the same concept ！！！ Total length = Maximum +1

Unsigned maximum calculation formula ：n byte =(2^n*8)-1

Both integer and point types can be constrained ：[UNSIGNED] [ZEROFILL]   Namely “ Unsigned ” and “ Zero compensation ”.

 Integer and point type syntax ： Basic grammar  [UNSIGNED] [ZEROFILL]
 UNSIGNED  Only non negative numbers can be stored 
 ZEROFILL  Automatically add zero in front of the stored value to fill the remaining digits , And automatically add UNSIGNED  attribute 

1.1 Examples of integer types

How to choose ？ The main Look at the effective range Whether it meets the business requirements

-- int  type , Unsigned , Not empty , Auto increment 
CREATE TABLE animals (
     id INT UNSIGNED NOT NULL AUTO_INCREMENT,
     name CHAR(30) NOT NULL,
     PRIMARY KEY (id)
);

--  Insert multiple data , No need to give id  assignment 
INSERT INTO animals (name)
 VALUES
('dog'),('cat'),('penguin'),('lax'),('whale'),('ostrich');

For general automatic increment ,int unsigned Can be stored 42 More than 100 million pieces of data , It's enough to use , No need to use bigint type . Use bigint Type means , The primary key of each data Need to use more than twice as much space and more computing resources .

1.2 Examples of point types

create table t1( id1 float(9,2), id2 double(9,2), id3 decimal(9,2));

 It can be seen that the grammar is the same 

insert into t1 values('1.2','1.24','1.235');

Meeting Automatically add decimal places 、 Beyond the decimal places rounding

among DECIMAL All basic calculations of columns (+、-、*、/) Are subject to 65 The accuracy of digits is completed . For storage Amount type , Be sure to use decimal Type to store .DECIMAL The maximum range is ：DECIMAL(65,30).

2. Character type

char Characteristics ：

• Save fixed length M

• The saved value is greater than M Will be cut off , Less than will be filled with spaces

• Maximum character length 255

varchar Characteristics ：

• Only store the actual length of the string

• Use extra 1~2 Bytes to store the length of the value （0~255 Use 1 byte , otherwise 2 byte ）

• Maximum character length 65535

Key points

• char and varchar The length of refers to “ Character length ”, Instead of byte length , Different encoding requires different byte lengths

• latin1 Occupy 1 Bytes ,gbk Occupy 2 Bytes ,utf8 Occupy 3 Bytes ,utf8mb4 Occupy 4 Bytes

• When using gbk When coding ,char The maximum character length is 255;varchar Maximum character length 32766

• When using utf8 When coding ,char The maximum character length is 255;varchar Maximum character length 21844

• When using utf8mb4 When coding ,char The maximum character length is 255;varchar Maximum character length 16383

Example ：

     Such as ：char(3), Fixed storage of three characters , Different codes Required for fixing Number of bytes ：latin1 need 3 Bytes ,gbk need 6 Bytes ,utf8 need 9 Bytes .

     Such as ：varchar(3), Store up to three characters , Different codes Maximum required Number of bytes ：latin1 need 4 Bytes ,gbk need 7 Bytes ,utf8 need 10 Bytes .

     Such as ：varchar(256), Maximum storage 256 Characters , Different codes Maximum required Number of bytes ：latin1 need 258 Bytes ,gbk need 514 Bytes ,utf8 need 770 Bytes .

-- grammar 
CHAR[(M)] [CHARACTER SET charset_name] [COLLATE collation_name]
VARCHAR(M) [CHARACTER SET charset_name] [COLLATE collation_name]
-- Or you could write it as 
CHAR[(M)] [CHARSET charset_name] [COLLATE collation_name]
VARCHAR(M) [CHARSET charset_name] [COLLATE collation_name]

How to choose char and varchar

     Fixed length option char, Not affected by coding . Such as ：uuid character string ,md3 character string ;

     Choose one that often changes length varchar. Such as ： Home address , Personal profile .

3. Time type

3.1 timestamp and datetime

See the examples in the table for the specific format of time type , among timestamp and datetime Can be accurate to seconds

Example

timestamp By default, it will be automatically updated to the current time ,datetime The default is empty. null, But you can specify the default value when you see the table , Make both the same default .

-- CURRENT_TIMESTAMP  Insert the default current time 
-- on update CURRENT_TIMESTAMP  The default time when updating 
create table time2(
  id int,
  ts timestamp,
  dt datetime default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP
);

timestamp and datetime  difference

• The number of bytes required is different ,timestamp 4 Bytes ,datetime 8 Bytes

• The effective range is different ,timestamp Maximum 2038 year ,datetime 9999 year

MySQL How to represent the current time in ？

In addition to the default values mentioned above CURRENT_TIMESTAMP

There are also the following

• CURRENT_TIMESTAMP()

• NOW()

• LOCALTIME

• LOCALTIME()

• LOCALTIMESTAMP

• LOCALTIMESTAMP()

Remember which one is simple , Remember function now() Just fine , Others look familiar .

3.2 date and time

There's nothing to talk about , Just remember the data format , See the table above for details

Look at an example

3.3 year

Year fixed 4 position , Let's look at an example

create table year1 ( id int,y year );insert into year1 (id,y) values (1,now()),(2,2022-12-30),(3,2022),(4,22);

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