作者：俊达
表数据乱码
表数据出现乱码的情况通常是由于数据的真实编码与相关参数不一致引起的，其中包括常见的参数如character_set_client、character_set_results、字段编码以及终端编码等。确保这些参数保持一致，可以有效预防和解决乱码问题。
测试1 - 表中的数据正常，但是character_set_result设置和终端的实际字符集不匹配
终端的字符编码为utf8
mysql> show variables like '%char%';
+--------------------------+------------------------------------------------------+
| Variable_name | Value |
+--------------------------+------------------------------------------------------+
| character_set_client | utf8mb3 |
| character_set_connection | utf8mb3 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8mb3 |
| character_set_server | latin1 |
| character_set_system | utf8mb3 |
| character_sets_dir | /usr/local/Cellar/mysql/8.0.31/share/mysql/charsets/ |
+--------------------------+------------------------------------------------------+
8 rows in set (0.00 sec)
mysql> create table test_charset(a varchar(100)) default charset utf8;
Query OK, 0 rows affected, 1 warning (0.01 sec)
mysql> insert into test_charset values('列列列列列AAA');
Query OK, 1 row affected (0.01 sec)
mysql> select * from test_charset;
+--------------------+
| a |
+--------------------+
| 列列列列列AAA |
+--------------------+
1 row in set (0.00 sec)
将character_set_results设置为gbk，则无法正常显示返回的数据：
mysql> set character_set_results=gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> select *,hex(a) from test_charset;
+---------------+--------------------------------------+
| a | hex(a) |
+---------------+--------------------------------------+
| ����������AAA | E58897E58897E58897E58897E58897414141 |
+---------------+--------------------------------------+
在另外一个GBK编码的终端下：
mysql> set character_set_results=gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> select *, hex(a) from test_charset;
+---------------+--------------------------------------+
| a | hex(a) |
+---------------+--------------------------------------+
| 列列列列列AAA | E58897E58897E58897E58897E58897414141 |
+---------------+--------------------------------------+
1 row in set (0.00 sec)
mysql> set character_set_results=utf8;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select *, hex(a) from test_charset;
+--------------------+--------------------------------------+
| a | hex(a) |
+--------------------+--------------------------------------+
| 鍒楀垪鍒楀垪鍒桝AA | E58897E58897E58897E58897E58897414141 |
+--------------------+--------------------------------------+
1 row in set (0.01 sec)
上面这几个例子中，数据库中存储的数据没有问题（使用hex函数查看真实的数据，和字符的字符集(UTF8)编码一致)，但是因为客户端的字符编码设置不对，导致无法正常查看字符。
测试2 - character_set_client和终端的字符集不匹配
2.1 utf8终端，character_set_client设置为gbk，写入数据
create table test_charset2(a varchar(100), b varchar(100)) default charset utf8;
-- utf8终端
mysql> set character_set_client='gbk';
Query OK, 0 rows affected (0.00 sec)
mysql> insert into test_charset2 values('term: utf8, charset client: gbk', '列列列列列AAA');
Query OK, 1 row affected (0.00 sec)
mysql> select *, hex(b) from test_charset2;
+---------------------------------+----------------------------+------------------------------------------------------+
| a | b | hex(b) |
+---------------------------------+----------------------------+------------------------------------------------------+
| term: utf8, charset client: gbk | 鍒楀垪鍒楀垪鍒桝AA | E98D92E6A580E59EAAE98D92E6A580E59EAAE98D92E6A19D4141 |
+---------------------------------+----------------------------+------------------------------------------------------+
1 row in set (0.00 sec)
汉字“列”的utf8编码应该是E58897，但是表中实际存储的数据不对。
>>> u"列".encode('utf8')
'\xe5\x88\x97'
测试2.1中，由于character_set_client和实际的数据的字符集编码不匹配，导致表里存储的数据本身就是有问题的（汉字“列”的utf8编码为e58897）。
使用python模拟这个过程：
-- 本来是utf8编码的数据，使用gbk编码来解码，然后再以utf8(表的字符集)方式编码
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8')
'\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa1\x9dAA'
>>> print u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8')
鍒楀垪鍒楀垪鍒桝AA
2.2 gbk终端，character_set_client设置为utf8，写入数据
-- gbk终端
mysql> set character_set_client='utf8';
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> insert into test_charset2 values('term: gbk, charset client: utf8', '列列列列列AAA');
Query OK, 1 row affected, 1 warning (0.01 sec)
mysql> show warnings;
+---------+------+-------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------+
| Warning | 1300 | Cannot convert string '\xC1\xD0\xC1\xD0\xC1\xD0...' from utf8mb3 to gbk |
+---------+------+-------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> select *, hex(b) from test_charset2;
+---------------------------------+--------------------+------------------------------------------------------+
| a | b | hex(b) |
+---------------------------------+--------------------+------------------------------------------------------+
| term: utf8, charset client: gbk | 鍒楀垪鍒楀垪鍒桝AA | E98D92E6A580E59EAAE98D92E6A580E59EAAE98D92E6A19D4141 |
| term: gbk, charset client: utf8 | ??????????AAA | 3F3F3F3F3F3F3F3F3F3F414141 |
+---------------------------------+--------------------+------------------------------------------------------+
2 rows in set (0.00 sec)
原因解析
本来时gbk方式编码的数据，使用utf8编码无法解码，使用替换字符（3F) 替换无法解码的数据。
2.3 utf8终端，character_set_results设置为gbk
如果我们将错就错，在utf8终端上，将character_set_results设置为gbk。
查询返回看起来正常的数据：
-- utf8编码的终端
mysql> set character_set_results=gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> select *, hex(b) from test_charset2;
+---------------------------------+--------------------+------------------------------------------------------+
| a | b | hex(b) |
+---------------------------------+--------------------+------------------------------------------------------+
| term: utf8, charset client: gbk | 列列列列列AAA | E98D92E6A580E59EAAE98D92E6A580E59EAAE98D92E6A19D4141 |
| term: gbk, charset client: utf8 | ??????????AAA | 3F3F3F3F3F3F3F3F3F3F414141 |
+---------------------------------+--------------------+------------------------------------------------------+
2 rows in set (0.00 sec)
原因解析
数据库中实际存储的数据：
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8')
'\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa1\x9dAA'
-- character_set_results 设置为gbk，则mysql以gbk编码返回数据，
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('gbk')
'\xe5\x88\x97\xe5\x88\x97\xe5\x88\x97\xe5\x88\x97\xe5\x88\x97AAA'
-- 由于终端字符集为utf8，以utf8编码解释返回的数据
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('gbk').decode('utf8')
u'\u5217\u5217\u5217\u5217\u5217AAA'
u5217就是汉字“列“的unicode编码
2.4 gbk终端，character_set_results设置为utf8
在gbk终端上，将character_set_results设置为utf8：
-- gbk编码的终端
mysql> set character_set_results=utf8;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select *, hex(b) from test_charset2;
+---------------------------------+----------------------------+------------------------------------------------------+
| a | b | hex(b) |
+---------------------------------+----------------------------+------------------------------------------------------+
| term: utf8, charset client: gbk | 閸掓鍨崚妤鍨崚妗滱A | E98D92E6A580E59EAAE98D92E6A580E59EAAE98D92E6A19D4141 |
| term: gbk, charset client: utf8 | ??????????AAA | 3F3F3F3F3F3F3F3F3F3F414141 |
+---------------------------------+----------------------------+------------------------------------------------------+
2 rows in set (0.00 sec)
原因解析
数据库中实际存储的数据：
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8')
'\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa1\x9dAA'
-- character_set_results 设置为utf8，则mysql以utf8编码返回数据，
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8')
'\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa5\x80\xe5\x9e\xaa\xe9\x8d\x92\xe6\xa1\x9dAA'
-- 由于终端字符集为gbk，以gbk编码解释返回的数据。
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8').decode('gbk')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'gbk' codec can't decode bytes in position 4-5: illegal multibyte sequence
-- 加上error='replace'
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8').decode('gbk', errors='replace')
u'\u95b8\u6393\ufffd\u9368\ufffd\u5d1a\u59a4\ufffd\u7059\u95b8\u6393\ufffdAA'
>>> print u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8').decode('gbk', errors='replace')
閸掓�鍨�崚妤�灙閸掓�AA
-- 使gb18030，
>>> u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8').decode('gb18030', errors='replace')
u'\u95b8\u6393\ue685\u9368\ue048\u5d1a\u59a4\ufffd\u7059\u95b8\u6393\ue522AA'
>>> print u"列列列列列AAA".encode('utf8').decode('gbk').encode('utf8').decode('gb18030', errors='replace')
閸掓鍨崚妤�灙閸掓AA
-- 上面的数据和mysql终端显示的数据（閸掓鍨崚妤鍨崚妗滱A ）有一些差异。原因可能是他们对gbk无法解码的数据处理方式有差异。
总结
当客户端字符编码方式和MySQL参数character_set_client、character_set_results设置不匹配时，容易产生乱码。有的时候，数据库存储的数据没有问题，但是客户端编码设置不对，导致看上去有乱码，这种情况下只需要将客户端编码设置正确，就可以了。有的时候，存储到数据库的数据就不对了，甚至原来的数据无法编码，会被“？”等替换字符替换，这时可能就无法得到原先的数据了。
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