Mysql——》varchar

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One 、varchar Introduce

VARCHAR The value in the column is variable length character string .VARCHAR The maximum effective length of is determined by the maximum line size and the character set used .

edition	varchar(n)	varchar(20)	Length range （ byte ）	Maximum length （ byte ）
MySQL 5.0.3 Before	n： Number of bytes	Maximum storage 20 Bytes utf-8 code ： You can only put 6 The Chinese characters	0-255	255
MySQL 5.0.3 after	n： Number of characters	Maximum storage 20 Characters It has nothing to do with coding	0-65535	65535： Specified maximum length 65533： When non empty fields are not allowed 65532： When non empty fields are allowed

1、 Why? varchar The maximum length is 65535 Bytes ？

Q： Why? varchar The maximum length is 65535 Bytes ？
A： because mysql It specifies that the data size of each row cannot exceed 65535 Bytes .

2、varchar Store the length in a few bytes ？

Q：varchar Store the length in a few bytes ？
A：
Less than 255 byte , use 1 Byte storage length
Greater than 255 byte , use 2 Byte storage length

3、varchar Use a few bytes to identify whether the field is empty

Q： A field, if allowed, is null, Identify with a few bytes ？
A： use 1 Bytes

Two 、varchar Maximum character length test

varchar Maximum character length = （ Maximum bytes per row （65535） - null Number of identification bytes - Length prefix bytes （1 or 2）） / The maximum number of bytes occupied by a single character

1、 scene

Create table ：test
Character set ：UTF8
Field ：name, Non empty , char(255)
Field ：address, Non empty ,varchar(n),n Is the maximum character length we will witness

2、 Calculation

The maximum number of bytes per row of data ：**65535 **
char(255) Maximum number of bytes ：255*3（ One character can occupy at most 3 Bytes ）
varchar(n) Storage length bytes ：2（ exceed 255 For byte 2 Byte storage length ）
The field is not empty ：0（ If the field allows null, You need to 1 Byte store null identification ）

varchar(n) The maximum number of bytes = **65535 - **255*3 - 2 = 64768
varchar(n) The maximum number of characters = Number of bytes /3
n = 64768/3 = 21589.33

3、 test

-- n=21589.33, Table created successfully 
create table test1 (name char(255) not null, address varchar(21589.33) not null );

-- n=21589.34, Table created successfully 
create table test2 (name char(255) not null, address varchar(21589.34) not null );

-- n=21589, Table created successfully 
create table test3 (name char(255) not null, address varchar(21589) not null );

-- n=21590, Failed to create table 
create table test4 (name char(255) not null, address varchar(21590) not null );

（1）n=21589.33, Table created successfully

（2）n=21589.34, Table created successfully

（3）n=21589, Table created successfully

（4）n=21590, Failed to create table

1118 - Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBs