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MySQL 字符集问题

2019-09-10 09:54:55??阅读:75??来源: 互联网

标签:ci?set?utf8mb4?character?字符集?问题?PAD?MySQL?Yes


MySQL的字符集和排序:

MySQL数据库的字符集包含两个基本概念:“字符集”和“排序”

“字符集”的英文是 character set,简称 charset。

“排序”的英文是 collate(动词),collation(名词)。

常见的字符集有:ascii、big5、bgk、latin1、utf8。

MySQL 5.3.3 中,加入了几个新的字符集:utf16、utf32、utf8mb4。

MySQL 中的 utf8 最多支持 3 字节的文字(实际上 utf8 是 utf8mb3 的别名),utf8mb4 最多支持 4 字节的文字。

“字符集”是数据库存储字符时用的编码格式,“排序”是使用 ORDER 或 WHERE a>100 等语句时,比较字符大小的方法。

utf8mb4_general_ci、utf8mb4_unicode_ci、utf8mb4_bin。

bin 指二进制比较,general 指比较字符串的编码值(实际比较的是 unicode 值)。

unicode 在 general 的基础上,增加了字母组合的处理,如德文中???和?ss?是相同的。

下段摘自https://dev.mysql.com/doc/refman/8.0/en/charset-unicode-sets.html

For any Unicode character set, operations performed using the?xxx_general_ci?collation are faster than those for the?xxx_unicode_ci?collation. For example, comparisons for the?utf8_general_ci?collation are faster, but slightly less correct, than comparisons for?utf8_unicode_ci. The reason is that?utf8_unicode_ci?supports mappings such as expansions; that is, when one character compares as equal to combinations of other characters. For example,???is equal to?ss?in German and some other languages.?utf8_unicode_ci?also supports contractions and ignorable characters.?utf8_general_ci?is a legacy collation that does not support expansions, contractions, or ignorable characters. It can make only one-to-one comparisons between characters.

查看数据库支持的字符集(MySQL 8.0):

mysql> show charset;

mysql> show character set; +----------+---------------------------------+---------------------+--------+ | Charset | Description | Default collation | Maxlen | +----------+---------------------------------+---------------------+--------+ | armscii8 | ARMSCII-8 Armenian | armscii8_general_ci | 1 | | ascii | US ASCII | ascii_general_ci | 1 | | big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 | | binary | Binary pseudo charset | binary | 1 | | cp1250 | Windows Central European | cp1250_general_ci | 1 | | cp1251 | Windows Cyrillic | cp1251_general_ci | 1 | | cp1256 | Windows Arabic | cp1256_general_ci | 1 | | cp1257 | Windows Baltic | cp1257_general_ci | 1 | | cp850 | DOS West European | cp850_general_ci | 1 | | cp852 | DOS Central European | cp852_general_ci | 1 | | cp866 | DOS Russian | cp866_general_ci | 1 | | cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 | | dec8 | DEC West European | dec8_swedish_ci | 1 | | eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 | | euckr | EUC-KR Korean | euckr_korean_ci | 2 | | gb18030 | China National Standard GB18030 | gb18030_chinese_ci | 4 | | gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 | | gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 | | geostd8 | GEOSTD8 Georgian | geostd8_general_ci | 1 | | greek | ISO 8859-7 Greek | greek_general_ci | 1 | | hebrew | ISO 8859-8 Hebrew | hebrew_general_ci | 1 | | hp8 | HP West European | hp8_english_ci | 1 | | keybcs2 | DOS Kamenicky Czech-Slovak | keybcs2_general_ci | 1 | | koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 | | koi8u | KOI8-U Ukrainian | koi8u_general_ci | 1 | | latin1 | cp1252 West European | latin1_swedish_ci | 1 | | latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 | | latin5 | ISO 8859-9 Turkish | latin5_turkish_ci | 1 | | latin7 | ISO 8859-13 Baltic | latin7_general_ci | 1 | | macce | Mac Central European | macce_general_ci | 1 | | macroman | Mac West European | macroman_general_ci | 1 | | sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 | | swe7 | 7bit Swedish | swe7_swedish_ci | 1 | | tis620 | TIS620 Thai | tis620_thai_ci | 1 | | ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 | | ujis | EUC-JP Japanese | ujis_japanese_ci | 3 | | utf16 | UTF-16 Unicode | utf16_general_ci | 4 | | utf16le | UTF-16LE Unicode | utf16le_general_ci | 4 | | utf32 | UTF-32 Unicode | utf32_general_ci | 4 | | utf8 | UTF-8 Unicode | utf8_general_ci | 3 | | utf8mb4 | UTF-8 Unicode | utf8mb4_0900_ai_ci | 4 | +----------+---------------------------------+---------------------+--------+ 41 rows in set (0.00 sec)

查看某个字符集支持的排序(MySQL 8.0):

mysql> show collation;
+----------------------------+----------+-----+---------+----------+---------+---------------+
| Collation                  | Charset  | Id  | Default | Compiled | Sortlen | Pad_attribute |
+----------------------------+----------+-----+---------+----------+---------+---------------+
| armscii8_bin               | armscii8 |  64 |         | Yes      |       1 | PAD SPACE     |

......      ......      ......      ......      ......      ......      ......      ..........

| utf8_vietnamese_ci         | utf8     | 215 |         | Yes      |       8 | PAD SPACE     |
+----------------------------+----------+-----+---------+----------+---------+---------------+
272 rows in set (0.00 sec)

latin1 支持的排序:

mysql> show collation like 'latin1%';
+-------------------+---------+----+---------+----------+---------+---------------+
| Collation         | Charset | Id | Default | Compiled | Sortlen | Pad_attribute |
+-------------------+---------+----+---------+----------+---------+---------------+
| latin1_bin        | latin1  | 47 |         | Yes      |       1 | PAD SPACE     |
| latin1_danish_ci  | latin1  | 15 |         | Yes      |       1 | PAD SPACE     |
| latin1_general_ci | latin1  | 48 |         | Yes      |       1 | PAD SPACE     |
| latin1_general_cs | latin1  | 49 |         | Yes      |       1 | PAD SPACE     |
| latin1_german1_ci | latin1  |  5 |         | Yes      |       1 | PAD SPACE     |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 | PAD SPACE     |
| latin1_spanish_ci | latin1  | 94 |         | Yes      |       1 | PAD SPACE     |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       1 | PAD SPACE     |
+-------------------+---------+----+---------+----------+---------+---------------+
8 rows in set (0.00 sec)

utf8mb4 支持的排序:

mysql> show collation like 'utf8mb4%';
+----------------------------+---------+-----+---------+----------+---------+---------------+
| Collation                  | Charset | Id  | Default | Compiled | Sortlen | Pad_attribute |
+----------------------------+---------+-----+---------+----------+---------+---------------+
| utf8mb4_0900_ai_ci         | utf8mb4 | 255 | Yes     | Yes      |       0 | NO PAD        |
| utf8mb4_0900_as_ci         | utf8mb4 | 305 |         | Yes      |       0 | NO PAD        |
| utf8mb4_0900_as_cs         | utf8mb4 | 278 |         | Yes      |       0 | NO PAD        |
| utf8mb4_0900_bin           | utf8mb4 | 309 |         | Yes      |       1 | NO PAD        |
| utf8mb4_bin                | utf8mb4 |  46 |         | Yes      |       1 | PAD SPACE     |
| utf8mb4_croatian_ci        | utf8mb4 | 245 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_cs_0900_ai_ci      | utf8mb4 | 266 |         | Yes      |       0 | NO PAD        |
| utf8mb4_cs_0900_as_cs      | utf8mb4 | 289 |         | Yes      |       0 | NO PAD        |
| utf8mb4_czech_ci           | utf8mb4 | 234 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_danish_ci          | utf8mb4 | 235 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_da_0900_ai_ci      | utf8mb4 | 267 |         | Yes      |       0 | NO PAD        |
| utf8mb4_da_0900_as_cs      | utf8mb4 | 290 |         | Yes      |       0 | NO PAD        |
| utf8mb4_de_pb_0900_ai_ci   | utf8mb4 | 256 |         | Yes      |       0 | NO PAD        |
| utf8mb4_de_pb_0900_as_cs   | utf8mb4 | 279 |         | Yes      |       0 | NO PAD        |
| utf8mb4_eo_0900_ai_ci      | utf8mb4 | 273 |         | Yes      |       0 | NO PAD        |
| utf8mb4_eo_0900_as_cs      | utf8mb4 | 296 |         | Yes      |       0 | NO PAD        |
| utf8mb4_esperanto_ci       | utf8mb4 | 241 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_estonian_ci        | utf8mb4 | 230 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_es_0900_ai_ci      | utf8mb4 | 263 |         | Yes      |       0 | NO PAD        |
| utf8mb4_es_0900_as_cs      | utf8mb4 | 286 |         | Yes      |       0 | NO PAD        |
| utf8mb4_es_trad_0900_ai_ci | utf8mb4 | 270 |         | Yes      |       0 | NO PAD        |
| utf8mb4_es_trad_0900_as_cs | utf8mb4 | 293 |         | Yes      |       0 | NO PAD        |
| utf8mb4_et_0900_ai_ci      | utf8mb4 | 262 |         | Yes      |       0 | NO PAD        |
| utf8mb4_et_0900_as_cs      | utf8mb4 | 285 |         | Yes      |       0 | NO PAD        |
| utf8mb4_general_ci         | utf8mb4 |  45 |         | Yes      |       1 | PAD SPACE     |
| utf8mb4_german2_ci         | utf8mb4 | 244 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_hr_0900_ai_ci      | utf8mb4 | 275 |         | Yes      |       0 | NO PAD        |
| utf8mb4_hr_0900_as_cs      | utf8mb4 | 298 |         | Yes      |       0 | NO PAD        |
| utf8mb4_hungarian_ci       | utf8mb4 | 242 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_hu_0900_ai_ci      | utf8mb4 | 274 |         | Yes      |       0 | NO PAD        |
| utf8mb4_hu_0900_as_cs      | utf8mb4 | 297 |         | Yes      |       0 | NO PAD        |
| utf8mb4_icelandic_ci       | utf8mb4 | 225 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_is_0900_ai_ci      | utf8mb4 | 257 |         | Yes      |       0 | NO PAD        |
| utf8mb4_is_0900_as_cs      | utf8mb4 | 280 |         | Yes      |       0 | NO PAD        |
| utf8mb4_ja_0900_as_cs      | utf8mb4 | 303 |         | Yes      |       0 | NO PAD        |
| utf8mb4_ja_0900_as_cs_ks   | utf8mb4 | 304 |         | Yes      |      24 | NO PAD        |
| utf8mb4_latvian_ci         | utf8mb4 | 226 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_la_0900_ai_ci      | utf8mb4 | 271 |         | Yes      |       0 | NO PAD        |
| utf8mb4_la_0900_as_cs      | utf8mb4 | 294 |         | Yes      |       0 | NO PAD        |
| utf8mb4_lithuanian_ci      | utf8mb4 | 236 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_lt_0900_ai_ci      | utf8mb4 | 268 |         | Yes      |       0 | NO PAD        |
| utf8mb4_lt_0900_as_cs      | utf8mb4 | 291 |         | Yes      |       0 | NO PAD        |
| utf8mb4_lv_0900_ai_ci      | utf8mb4 | 258 |         | Yes      |       0 | NO PAD        |
| utf8mb4_lv_0900_as_cs      | utf8mb4 | 281 |         | Yes      |       0 | NO PAD        |
| utf8mb4_persian_ci         | utf8mb4 | 240 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_pl_0900_ai_ci      | utf8mb4 | 261 |         | Yes      |       0 | NO PAD        |
| utf8mb4_pl_0900_as_cs      | utf8mb4 | 284 |         | Yes      |       0 | NO PAD        |
| utf8mb4_polish_ci          | utf8mb4 | 229 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_romanian_ci        | utf8mb4 | 227 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_roman_ci           | utf8mb4 | 239 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_ro_0900_ai_ci      | utf8mb4 | 259 |         | Yes      |       0 | NO PAD        |
| utf8mb4_ro_0900_as_cs      | utf8mb4 | 282 |         | Yes      |       0 | NO PAD        |
| utf8mb4_ru_0900_ai_ci      | utf8mb4 | 306 |         | Yes      |       0 | NO PAD        |
| utf8mb4_ru_0900_as_cs      | utf8mb4 | 307 |         | Yes      |       0 | NO PAD        |
| utf8mb4_sinhala_ci         | utf8mb4 | 243 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_sk_0900_ai_ci      | utf8mb4 | 269 |         | Yes      |       0 | NO PAD        |
| utf8mb4_sk_0900_as_cs      | utf8mb4 | 292 |         | Yes      |       0 | NO PAD        |
| utf8mb4_slovak_ci          | utf8mb4 | 237 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_slovenian_ci       | utf8mb4 | 228 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_sl_0900_ai_ci      | utf8mb4 | 260 |         | Yes      |       0 | NO PAD        |
| utf8mb4_sl_0900_as_cs      | utf8mb4 | 283 |         | Yes      |       0 | NO PAD        |
| utf8mb4_spanish2_ci        | utf8mb4 | 238 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_spanish_ci         | utf8mb4 | 231 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_sv_0900_ai_ci      | utf8mb4 | 264 |         | Yes      |       0 | NO PAD        |
| utf8mb4_sv_0900_as_cs      | utf8mb4 | 287 |         | Yes      |       0 | NO PAD        |
| utf8mb4_swedish_ci         | utf8mb4 | 232 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_tr_0900_ai_ci      | utf8mb4 | 265 |         | Yes      |       0 | NO PAD        |
| utf8mb4_tr_0900_as_cs      | utf8mb4 | 288 |         | Yes      |       0 | NO PAD        |
| utf8mb4_turkish_ci         | utf8mb4 | 233 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_unicode_520_ci     | utf8mb4 | 246 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_unicode_ci         | utf8mb4 | 224 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_vietnamese_ci      | utf8mb4 | 247 |         | Yes      |       8 | PAD SPACE     |
| utf8mb4_vi_0900_ai_ci      | utf8mb4 | 277 |         | Yes      |       0 | NO PAD        |
| utf8mb4_vi_0900_as_cs      | utf8mb4 | 300 |         | Yes      |       0 | NO PAD        |
| utf8mb4_zh_0900_as_cs      | utf8mb4 | 308 |         | Yes      |       0 | NO PAD        |
+----------------------------+---------+-----+---------+----------+---------+---------------+
75 rows in set (0.00 sec)

上表中可以看出,latin1 的默认排序是 latin1_swedish_ci,utf8mb4 的默认排序是 utf8mb4_0900_ai_ci。

对于字符集的设置,有几个级别:

  • 数据库程序实例级别(指一个运行着的数据库进程)
  • 数据库级别
  • 数据表级别
  • 数据列级别
  • SQL 语句级别

优先级依次递增

对于数据库实例级别的设置,有两个方法:

1、修改 my.cnf 文件:

character_set_server=utf8mb4
; 如果没设置 character_set_server,MySQL 8.0 之前默认是 latin1,MySQL 8.0 及之后默认是 utf8mb4
collation=utf8mb4_0900_ai_ci
; 如果没设置 collation,则使用字符集的默认排序
; latin1 的默认排序是 latin1_swedish_ci,utf8mb4 的默认排序是 utf8mb4_0900_ai_ci

?2、启动服务时设定

mysqld --character-set-server=utf8mb4 --collation-server=utf8mb4_0900_ai_ci

这两个我都没试过,因为新建数据库和新建数据表时,最好显式提供 character set 和 collation,数据库实例级别的设置对我们没有任何影响。

?对于数据库级别的设置(MySQL 5.7):

?下例:MySQL 5.7 默认字符集是 latin1,新建数据库时,不指定字符集,便使用默认字符集 latin1。

mysql> create database mytest;
Query OK, 1 row affected

mysql> show create database mytest;
+----------+-------------------------------------------------------------------+
| Database | Create Database                                                   |
+----------+-------------------------------------------------------------------+
| mytest   | CREATE DATABASE `mytest` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+-------------------------------------------------------------------+
1 row in set

mysql> drop database mytest;
Query OK, 0 rows affected

下例:新建数据库时,指定字符集为 utf8mb4。

mysql> create database mytest default character set utf8mb4;
Query OK, 1 row affected

mysql> show create database mytest;
+----------+--------------------------------------------------------------------+
| Database | Create Database                                                    |
+----------+--------------------------------------------------------------------+
| mytest   | CREATE DATABASE `mytest` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+--------------------------------------------------------------------+
1 row in set

mysql> drop database mytest;
Query OK, 0 rows affected

下例:新建数据库时,指定字符集为 utf8mb4,指定排序为 utf8mb4_general_ci(MySQL 8.0 之前,没有?utf8mb4_0900_ai_ci)。

数据库 DDL 不显示默认排序规则。

mysql> create database mytest default character set utf8mb4 collate utf8mb4_general_ci;
Query OK, 1 row affected

mysql> show create database mytest;
+----------+--------------------------------------------------------------------+
| Database | Create Database                                                    |
+----------+--------------------------------------------------------------------+
| mytest   | CREATE DATABASE `mytest` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+--------------------------------------------------------------------+
1 row in set

mysql> drop database mytest;
Query OK, 0 rows affected

下例:新建数据库时,指定字符集为 utf8mb4,指定排序为 utf8mb4_bin。

mysql> create database mytest default character set utf8mb4 collate utf8mb4_bin;
Query OK, 1 row affected

mysql> show create database mytest;
+----------+----------------------------------------------------------------------------------------+
| Database | Create Database                                                                        |
+----------+----------------------------------------------------------------------------------------+
| mytest   | CREATE DATABASE `mytest` /*!40100 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin */ |
+----------+----------------------------------------------------------------------------------------+
1 row in set

mysql> drop database mytest;
Query OK, 0 rows affected

?下例:修改已有数据库的字符集和排序。

mysql> alter database mytest default character set utf8mb4;
Query OK, 1 row affected

mysql> alter database mytest collate utf8mb4_bin;
Query OK, 1 row affected

mysql> alter database mytest default character set utf8mb4 collate utf8mb4_bin;
Query OK, 1 row affected

数据表级别

TODO

数据列级别

TODO

SQL 语句级别

TODO

?

?数据库中字符集相关的几个变量:

?

MySQL [(none)]> show variables like '%char%';                                   mysql> show variables like '%char%';
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
| Variable_name            | Value                                         |    | Variable_name            | Value                                         |
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
| character_set_client     | utf8                                          |    | character_set_client     | gbk                                           |
| character_set_connection | utf8                                          |    | character_set_connection | gbk                                           |
| character_set_database   | latin1                                        |    | character_set_database   | latin1                                        |
| character_set_filesystem | binary                                        |    | character_set_filesystem | binary                                        |
| character_set_results    | utf8                                          |    | character_set_results    | gbk                                           |
| character_set_server     | latin1                                        |    | character_set_server     | latin1                                        |
| character_set_system     | utf8                                          |    | character_set_system     | utf8                                          |
| character_sets_dir       | d:\wamp\bin\mysql\mysql5.7.24\share\charsets\ |    | character_sets_dir       | d:\wamp\bin\mysql\mysql5.7.24\share\charsets\ |
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
8 rows in set, 1 warning (0.006 sec)                                            8 rows in set, 1 warning (0.01 sec)

MySQL [(none)]> use mytest;                                                     mysql> use mytest;
Database changed                                                                Database changed
                                                                                
MySQL [mytest]> show variables like '%char%';                                   mysql> show variables like '%char%';
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
| Variable_name            | Value                                         |    | Variable_name            | Value                                         |
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
| character_set_client     | utf8                                          |    | character_set_client     | gbk                                           |
| character_set_connection | utf8                                          |    | character_set_connection | gbk                                           |
| character_set_database   | utf8mb4                                       |    | character_set_database   | utf8mb4                                       |
| character_set_filesystem | binary                                        |    | character_set_filesystem | binary                                        |
| character_set_results    | utf8                                          |    | character_set_results    | gbk                                           |
| character_set_server     | latin1                                        |    | character_set_server     | latin1                                        |
| character_set_system     | utf8                                          |    | character_set_system     | utf8                                          |
| character_sets_dir       | d:\wamp\bin\mysql\mysql5.7.24\share\charsets\ |    | character_sets_dir       | d:\wamp\bin\mysql\mysql5.7.24\share\charsets\ |
+--------------------------+-----------------------------------------------+    +--------------------------+-----------------------------------------------+
8 rows in set, 1 warning (0.007 sec)                                            8 rows in set, 1 warning (0.00 sec)

?上例:

使用MySQL 5.7。

左侧展示使用 Cygwin 连接的操作结果,右侧展示使用 cmd 连接的操作结果。

可以看出:?character_set_client?、?character_set_connection?、?character_set_results ?三个变量,跟客户端使用的字符集有关。Cygwin 使用 utf8 字符集,这三个变量就是 utf8,cmd 使用 gbk 字符集,这三个变量就是 bgk。

?character_set_server ?变量:此变量是数据库实例级别的字符集设置值。

?character_set_database ?变量:此变量是数据库级别的字符集设置值。此实例中,第一次执行?show variables like '%char%';?时,还未选择任何数据库,此变量值默认等于??character_set_server ?,即?latin1?。选择?mytest?数据库后,此变量变为?mytest?数据库的字符集设置,即?utf8mb4?。

乱码是怎样产生的(个人理解)

为方便说明,以下提到字符集时,同一个数据库中,数据表、数据列使用与数据库相同的字符集。

数据库相关的乱码,产生的时机有两个:一是往数据库中存储时产生,二是从数据库中取出时产生。

数据库为保证内部编码的正确性,使客户端的字符集与服务端的字符集无关。

大体流程是:

写入数据库时:

  1. 通过 ?character_set_client ?判断客户端使用的字符集?charset_from
  2. 通过?character_set_server?、?character_set_database?、数据表、数据列、SQL语句的优先级,确定数据存储时使用的字符集charset_to
  3. 把客户端传入的字符数据,由charset_from?转换为charset_to?,存储在数据库文件中

读取数据库时:

  1. 通过?character_set_server?、?character_set_database?、数据库、数据列、SQL语句的优先级,确定数据存储时使用的字符集?charset_from?
  2. 通过?character_set_results?(注意不是?character_set_client?)判断客户端希望收到的字符集?charset_to?
  3. 把查询结果中的字符数据,由?charset_from?转换为?charset_to?,返回给客户端

从流程上可以看出,客户端使用什么样的字符集,与数据库使用什么样的字符集,是无关的(不考虑转换不成功不能存储的问题)。

只要数据库设置为 utf8mb4(可以存储所有现有字符),客户端??set names ******;?设置好客户端使用的字符集, 不存在乱码问题。

现在主流版本的 MySQL(服务端和客户端配合使用),在存入字符时,会做字符有效性检查,在一定程度上保证存入字符的正确性。下例左侧服务端和客户端为 MySQL 5.0.77,右侧服务端和客户端为 MySQL 5.7.24:

客户端均使用cmd(gbk)
mysql> show variables like 'version'; mysql> show variables like 'version'; +---------------+------------------+ +---------------+--------+ | Variable_name | Value | | Variable_name | Value | +---------------+------------------+ +---------------+--------+ | version | 5.0.77-community | | version | 5.7.24 | +---------------+------------------+ +---------------+--------+ 1 row in set (0.00 sec) 1 row in set, 1 warning (0.01 sec) mysql> create database mytest default character set latin1; mysql> create database mytest default character set latin1; Query OK, 1 row affected (0.00 sec) Query OK, 1 row affected (0.00 sec) mysql> use mytest; mysql> use mytest; Database changed Database changed mysql> create table mytable (name text); mysql> create table mytable (name text); Query OK, 0 rows affected (0.02 sec) Query OK, 0 rows affected (0.07 sec) mysql> insert into mytable (name) values ('中文'); mysql> insert into mytable (name) values ('中文'); Query OK, 1 row affected (0.00 sec) ERROR 1366 (HY000): Incorrect string value: '\xD6\xD0\xCE\xC4' for column 'name' at row 1 mysql> select * from mytable; mysql> select * from mytable; +------+ Empty set (0.00 sec) | name | +------+ | 中文 | +------+ 1 row in set (0.00 sec)

?

左侧的 MySQL 5.0.77,可以存入中文,可以取出中文。

但是,存入的字符串是从?gbk?转为?latin1?(实际是不能转换成功的,个人认为这种转换是将每个汉字的两个 gbk 字节转为两个单字节存入数据库),以乱码的方式存到数据库当中的(转换后的单字节很多不存在于 latin1 字符集)。

取出来时,又进行了一次转换(每遇到两个字节,转换为一个 gbk 字符)。由于客户端使用 gbk 字符集,所以碰巧显示成功。

使用 Cygwin(utf8)读取一下刚才存入的中文:

MySQL [mytest]> select * from mytable;
+----------+
| name     |
+----------+
| ?D??     |
+----------+
1 row in set (0.000 sec)

?




我曾经以为,使用不同字符编码的客户端,操作同一个数据库,会出现各种问题。

直到最近才搞清楚,数据库设置好(需要存储中文的库、表、列,设置为?utf8mb4?等可以存储所有字符的字符集),客户端搞清楚自己使用的字符编码(自己是?gbk?就不要非把自己设置为?utf8?,hold 不住),服务端和客户端各司其职,将不会出现乱码问题。

?



标签:ci,set,utf8mb4,character,字符集,问题,PAD,MySQL,Yes

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