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MySQL character type learning notes

2022-07-05 10:04:00 【51CTO】

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One 、 Character set and character encoding

1.1、 Character set

Character set ： A character set is a set of characters , for example GB2312 Is the simplified Chinese character set of Chinese national standard ,GB2312 Include simplified Chinese characters （6763 individual ） And general symbols 、 Serial number 、 Numbers 、 Latin alphabet 、 Japanese kana 、 The Greek letter 、 Russian letters 、 Chinese pinyin symbols 、 Chinese phonetic alphabet , common 7445 Graphic characters .

Character set query sql：

      
      
       
       #  Method 1 ： direct show charset
       
       
SHOW CHARSET;
       
       
 # Method 2 ： Inquire about information_schema Example of character_sets
       
       
SELECT * FROM information_schema.character_sets;
      
      
      
      
       
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I'm a local 5.7.22 edition , The supported character sets are as follows ：

Charset	Description	Default collation	Maxlen
big5	Big5 Traditional Chinese	big5_chinese_ci	2
dec8	DEC West European	dec8_swedish_ci	1
cp850	DOS West European	cp850_general_ci	1
hp8	HP West European	hp8_english_ci	1
koi8r	KOI8-R Relcom Russian	koi8r_general_ci	1
latin1	cp1252 West European	latin1_swedish_ci	1
latin2	ISO 8859-2 Central European	latin2_general_ci	1
swe7	7bit Swedish	swe7_swedish_ci	1
ascii	US ASCII	ascii_general_ci	1
ujis	EUC-JP Japanese	ujis_japanese_ci	3
sjis	Shift-JIS Japanese	sjis_japanese_ci	2
hebrew	ISO 8859-8 Hebrew	hebrew_general_ci	1
tis620	TIS620 Thai	tis620_thai_ci	1
euckr	EUC-KR Korean	euckr_korean_ci	2
koi8u	KOI8-U Ukrainian	koi8u_general_ci	1
gb2312	GB2312 Simplified Chinese	gb2312_chinese_ci	2
greek	ISO 8859-7 Greek	greek_general_ci	1
cp1250	Windows Central European	cp1250_general_ci	1
gbk	GBK Simplified Chinese	gbk_chinese_ci	2
latin5	ISO 8859-9 Turkish	latin5_turkish_ci	1
armscii8	ARMSCII-8 Armenian	armscii8_general_ci	1
utf8	UTF-8 Unicode	utf8_general_ci	3
ucs2	UCS-2 Unicode	ucs2_general_ci	2
cp866	DOS Russian	cp866_general_ci	1
keybcs2	DOS Kamenicky Czech-Slovak	keybcs2_general_ci	1
macce	Mac Central European	macce_general_ci	1
macroman	Mac West European	macroman_general_ci	1
cp852	DOS Central European	cp852_general_ci	1
latin7	ISO 8859-13 Baltic	latin7_general_ci	1
utf8mb4	UTF-8 Unicode	utf8mb4_general_ci	4
cp1251	Windows Cyrillic	cp1251_general_ci	1
utf16	UTF-16 Unicode	utf16_general_ci	4
utf16le	UTF-16LE Unicode	utf16le_general_ci	4
cp1256	Windows Arabic	cp1256_general_ci	1
cp1257	Windows Baltic	cp1257_general_ci	1
utf32	UTF-32 Unicode	utf32_general_ci	4
binary	Binary pseudo charset	binary	1
geostd8	GEOSTD8 Georgian	geostd8_general_ci	1
cp932	SJIS for Windows Japanese	cp932_japanese_ci	2
eucjpms	UJIS for Windows Japanese	eucjpms_japanese_ci	3
gb18030	China National Standard GB18030	gb18030_chinese_ci	4

1.2、 Character encoding

Character encoding ： Character encoding is to map characters to specific bytes or byte sequences , But generally, a specific character set adopts a specific encoding method

Character encoding query sql：

      
      
       
       # Method 1 ： direct show collation
       
       
SHOW COLLATION;
       
       
# Method 2 ： Inquire about information_schema Of collations
       
       
SELECT * FROM information_schema.collations;
      
      
      
      
       
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Check the character code , The following is 5.7.22 Version of , Yes 222 Bar record ：

MySQL Character type learning notes _ Character encoding

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Be careful ：MySQL The whole table can be set if the character set of is not set , It can also be detailed to specific fields , The usage is to add charset [ Character set name ]

Two 、 Character set collation

2.1、 Collation definition

Sort rule (Collation)： Sorting rules refer to the comparison rules of characters under character set .

2.2 、 Collation features

Collation features ：

Each character set has a default collation
A character set corresponds to a collation , Two different character sets cannot have the same collation

Be careful ： The common naming rules for sorting rules are ： With _ci The ending is not case sensitive (case insensitive), With _cs The ending is case sensitive (case sensitive), With _bin The ending represents a binary comparison (binary)

Pictured , Choose one. collation,5.7.22 Version of , Most of them are ci At the end of the , That is, the case is not sensitive

MySQL Character type learning notes _sql_02

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Example ：

      
      
       
       #  Build table , Character encoding defaults to utf8_general_ci
       
       
 CREATE TABLE t (
       
       
a VARCHAR(10)
       
       
 )CHARSET = utf8;
       
       
 #  Write two pieces of data 
       
       
 INSERT INTO t SELECT 'a';
       
       
 INSERT INTO t SELECT 'A';
       
       
 #  Find out two pieces of data , It means that case is not sensitive 
       
       
SELECT * FROM t WHERE a='a';
       
       
#  The only index can't be built 
       
       
ALTER TABLE t ADD UNIQUE KEY (a);
      
      
      
      
       
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       #  Change the character encoding 
       
       
ALTER TABLE t MODIFY COLUMN a VARCHAR(10) COLLATE utf8_bin;
       
       
#  Find a piece of data , Show case sensitivity 
       
       
SELECT * FROM t WHERE a = 'a';
       
       
#  This situation   The only index that can be built 
       
       
ALTER TABLE t ADD UNIQUE KEY (a);
      
      
      
      
       
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3、 ... and 、CHAR and VARCHAR

char and varchar Is the two most common string types , The grammar is char(N) and varchar(N), Pay attention to N stay MySQL4.1 After the version, it indicates the length of the characters , Not byte length , stay MySQL4.1 The previous version only represents the length of bytes

3.1、CHAR type

about CHAR(N),N For the range of 0~255

CHAR(N) To save a fixed length string , Also according to the set value N, If N Set to 10, No matter what range of strings you pass , It's all fixed length 10 Of , Because the database populates the right side of the storage column (Right padded), When reading, the database will automatically delete the filled characters . Unless set SQL_MODE attribute PAD_CHAR_TO_FULL_LENGTH, Can display normally

Example practice ：

      
      
       
       #  Practice of building watch 
       
       
CREATE TABLE t (a CHAR(10));
       
       
#  Write data 
       
       
INSERT INTO t SELECT 'abcd';
       
       
#  Inquire about 
       
       
SELECT a, HEX(a), LENGTH(a), CHAR_LENGTH(a) FROM t;
      
      
      
      
       
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MySQL Character type learning notes _ Character encoding _03

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       #  Query Global SQL_MODE
       
       
SELECT @@global.sql_mode;
       
       
#  Query session SQL_MODE
       
       
SELECT @@session.sql_mode; 
       
       
#  Set up SQL_MODE attribute 
       
       
SET SQL_MODE = 'PAD_CHAR_TO_FULL_LENGTH';
       
       
#  Query again 
       
       
SELECT a, HEX(a), LENGTH(a), CHAR_LENGTH(a) FROM t;
      
      
      
      
       
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MySQL Character type learning notes _ Sort rule _04

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3.2、VARCHAR type

about varchar Type is used to store variable length strings , It means varchar(10) The column of , I write strings “abcd“, This column stores 4 Characters , and char(10) The columns of are stored 10 Characters

Four 、BINARY and VARBINARY

4.1、 Feature comparison

BINARY(N) and VARBINARY(N) And CHAR(N) and VARCHAR(N) The type is a bit similar ,BINARY Analogical CHAR,VARBINARY By analogy VARCHAR type .

contrast ：
The difference is BINARY and VARBINARY All of them are binary strings ,CHAR and VARCHAR What is stored is a string of character type
BINARY and VARBINARY There is no concept of character set ,CHAR and VARCHAR These are the concepts of character sets
BINARY(N) and VARBINARY(N) Medium N The length of the bytes represented , We know from the front that CHAR(N) and VARCHAR(N) Medium N since MySQL4.1 after , It means the length of the character
CHAR and VARCHAR When comparing characters , Is to compare the characters stored in itself , Ignore filled characters , and BINARY and VARBINARY Is not ignored , The comparison is binary

4.2、 Example practice

      
      
       
       SELECT 
       
       
  HEX('a'),
       
       
  HEX('a '),
       
       
  'a' = 'a ',
       
       
  BINARY('a') = BINARY('a ') ;
      
      
      
      
       
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MySQL Character type learning notes _ Character encoding _05

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5、 ... and 、BLOB and TEXT

Can be blob A column of type is considered large enough varbinary Column of type , Can also be text A column of type is considered large enough varchar Column of type

5.1、text type

TEXT It is used to store character big data type ,TEXT The type and blob Same type , Can be subdivided into ：

tinytext(2^8)
text(2^16)
mediumtext(2^24)
longtext(2^32)

5.2、blob type

BLOB(Binary Large Object) It is used to store binary big data type . Depending on the storage length ,blob The types can be subdivided into ：

tinyblob(2^8)
blob(2^16)
mediumblob(2^24)
longblob(2^32)

5.3、 Scheduling problem

blob and text Scheduling problem ：
Be careful ： BLOB and TEXT Only the first... Of the column is used for sorting max_sort_length Bytes

      
      
       
       SET GLOBAL max_sort_length =1024;
       
       
SELECT @@global.max_sort_length;
      
      
      
      
       
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5.4、 Index problem

blob and text The question of the Picasso ：
blob and text Type of column plus index , Need to pay attention to some details

1、 There cannot be default values for columns
2、 You need to specify the length of index prefix when adding index

      
      
       
       CREATE INDEX indexName ON tablename(columnTest(1000));
      
      
      
      
       
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6、 ... and 、ENUM and SET type

6.1、 Introduction to set types

enum and set All types are set types , The difference is enum Enumerate at most 65536 Elements , and set Type enumeration at most 64 Elements

6.2、 Examples of set types

      
      
       
       #  Create table validation problem 
       
       
CREATE TABLE t (username VARCHAR(20),sex ENUM('male','female'))ENGINE=INNODB;
       
       
#  Write data normally 
       
       
INSERT INTO t SELECT 'David','male';
       
       
INSERT INTO t SELECT 'john','female';
       
       
#  Writing data , Because there is no corresponding enumeration type , But it can still write , It's not a warning 
       
       
INSERT INTO t SELECT 'mariah','security';
       
       
#  Find out , Find out sex Field is not worth 
       
       
select * from t ;
       
       
#  Set strict mode 
       
       
SET SQL_MODE = 'strict_trans_tables';
       
       
#  Write the data again , You can't write 
       
       
INSERT INTO t SELECT 'mariah','security';
       
       
#  Query again failed to write successfully 
       
       
SELECT * FROM t;
      
      
      
      
       
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