[MySQL string data type optimization] the difference between char and varchar

Preface

Have studied MySQL Friends of the database must be right

char and varchar

These two string data types are not new , So these two types of

difference 、 Details and usage scenarios

Do you know ？

char And varchar contrast

• char(size),
  Fixed length
  character string , Maximum 255 character .

• varchar(size),
  Variable length
  character string , Maximum ==65535 byte ==( Notice that here are bytes , Characters occupy different bytes in different encoding rules ), If the table code is not defined
  The default is utf8
  【utf8 Maximum coding 21844 character ;gdk Code up to 32766 character 】

• ( Pay attention to characters, not bytes )

• char Assign fixed characters ,char(4) Occupy four characters , Save a character , It will also occupy four characters in memory .

• varchar
  Allocate space according to actual use
  , Still need to use 1-3 Bytes to record the length of the stored content ,
  Total occupancy  ＝ L（ The actual data size ）+（1-3） Bytes
  .

Code samples

CREATE TABLE t4(
 `name` CHAR(255));
# Create a table t4, Definition name The length of is 255 character
INSERT INTO t4 VALUES('ABCD');
INSERT INTO t4 VALUES(' I am you D');
# Add English... To the table 、 Chinese is OK
CREATE TABLE t5(
 `name` VARCHAR(32766)) CHARSET gbk;
# Define code as gdk, Maximum length is 32766 character , because gbk Chinese accounts for 2 Bytes , English 1 Bytes
CREATE TABLE t6(
 `name` VARCHAR(21844));
 # Define the code as the default utf8, Maximum length is 21844 character , because utf-8 Chinese accounts for 3 Bytes , English 1 Bytes


char And varchar Details of the use of

• char(4), This 4 Indicates the number of characters , Non byte number , There are four letters in both Chinese and English （ The occupied character space is fixed ）.

• varchar(4), This 4 Also represents the number of characters , But the letters or Chinese characters stored here are
  Calculate according to the code when defining the table
  （ The occupied character space is not fixed ）.

char and varchar The use of

• Fixed data length , recommend char
  . for example md5 password （ Fix 32 by ）、 Zip code 、 cell-phone number 、 ID number, etc .2.
    The length is uncertain , Use varchar
  . E.g. message 、 Articles, etc .