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在DB2数据库中，字符/字符串比较的时候，默认是区分大小写的，即'abc'和'ABC'是两个不同的字符串，但在 MySQL中，默认是不区分大小写的，因此迁移的时候需要注意。
示例1中，可以看到'abc' 和 'ABC' 在innodb中存放的数据是不一样的，但比较的时候认为是一样的
mysql> create table mqs1(col1 char(20)) CHARSET=utf8mb4;
Query OK, 0 rows affected (0.26 sec)
mysql> insert into mqs1 values('abc'),('aBC'),('ABC'),('AbC');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from mqs1 where col1='abc';
+------+
| col1 |
+------+
| abc |
| aBC |
| ABC |
| AbC |
+------+
4 rows in set (0.00 sec)
mysql> select HEX(col1) from mqs1;
+-----------+
| HEX(col1) |
+-----------+
| 616263 |
| 614243 |
| 414243 |
| 416243 |
+-----------+
4 rows in set (0.00 sec)
出现以上现象的原因是对于一个字段有 charset 和 collate 两个属性，字符比较时是根据它的 collate 来决定的。
可以使用 show collation 来看一个 charset 有哪些 collate
mysql> show collation where charset='utf8mb4';
+------------------------+---------+-----+---------+----------+---------+
| Collation | Charset | Id | Default | Compiled | Sortlen |
+------------------------+---------+-----+---------+----------+---------+
| utf8mb4_general_ci | utf8mb4 | 45 | Yes | Yes | 1 |
| utf8mb4_bin | utf8mb4 | 46 | | Yes | 1 |
| utf8mb4_unicode_ci | utf8mb4 | 224 | | Yes | 8 |
| utf8mb4_icelandic_ci | utf8mb4 | 225 | | Yes | 8 |
| utf8mb4_latvian_ci | utf8mb4 | 226 | | Yes | 8 |
| utf8mb4_romanian_ci | utf8mb4 | 227 | | Yes | 8 |
| utf8mb4_slovenian_ci | utf8mb4 | 228 | | Yes | 8 |
| utf8mb4_polish_ci | utf8mb4 | 229 | | Yes | 8 |
| utf8mb4_estonian_ci | utf8mb4 | 230 | | Yes | 8 |
| utf8mb4_spanish_ci | utf8mb4 | 231 | | Yes | 8 |
| utf8mb4_swedish_ci | utf8mb4 | 232 | | Yes | 8 |
| utf8mb4_turkish_ci | utf8mb4 | 233 | | Yes | 8 |
| utf8mb4_czech_ci | utf8mb4 | 234 | | Yes | 8 |
| utf8mb4_danish_ci | utf8mb4 | 235 | | Yes | 8 |
| utf8mb4_lithuanian_ci | utf8mb4 | 236 | | Yes | 8 |
| utf8mb4_slovak_ci | utf8mb4 | 237 | | Yes | 8 |
| utf8mb4_spanish2_ci | utf8mb4 | 238 | | Yes | 8 |
| utf8mb4_roman_ci | utf8mb4 | 239 | | Yes | 8 |
| utf8mb4_persian_ci | utf8mb4 | 240 | | Yes | 8 |
| utf8mb4_esperanto_ci | utf8mb4 | 241 | | Yes | 8 |
| utf8mb4_hungarian_ci | utf8mb4 | 242 | | Yes | 8 |
| utf8mb4_sinhala_ci | utf8mb4 | 243 | | Yes | 8 |
| utf8mb4_german2_ci | utf8mb4 | 244 | | Yes | 8 |
| utf8mb4_croatian_ci | utf8mb4 | 245 | | Yes | 8 |
| utf8mb4_unicode_520_ci | utf8mb4 | 246 | | Yes | 8 |
| utf8mb4_vietnamese_ci | utf8mb4 | 247 | | Yes | 8 |
+------------------------+---------+-----+---------+----------+---------+
26 rows in set (0.00 sec)
可以看到 utf8mb4 默认的 collation 是 utf8mb4_general_ci， 最后的 "_ci" 表示 case insensitive (大小写不敏感)
那么如何调整为大小写敏感呢？可以看到 utf8mb4 对应的 Collation 只有一个不是 "_ci" 结尾的，即 utf8mb4_bin， 那只能试一下了，发现可以区分大小写：
mysql> create table mqs3(col1 char(20) not null primary key) CHARSET utf8mb4 collate utf8mb4_bin;
Query OK, 0 rows affected (0.07 sec)
mysql> insert into mqs3 values('abc'),('aBC'),('ABC'),('AbC');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from mqs3 where col1='abc';
+------+
| col1 |
+------+
| abc |
+------+
1 row in set (0.00 sec)
mysql> show create table mqs3 \G
*************************** 1. row ***************************
Table: mqs3
Create Table: CREATE TABLE "mqs3" (
"col1" char(20) COLLATE utf8mb4_bin NOT NULL,
PRIMARY KEY ("col1")
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
1 row in set (0.00 sec)
注意一点： 修改表的 COLLATE 对已经存在的列不生效，主要原因是在创建表的时候，COLLATE 既可以是某一个字段的选项，也可以是整个表的选项，两者是相对独立的。 因此修改表的 COLLATE 不影响已有字段的定义。?
事实上，同一张表的不同的字段可以有不同的 COLLATE，示例如下：
mysql> create table mqs4(col1 char(20) COLLATE utf8_general_ci, col2 char(20) COLLATE utf8_bin);
Query OK, 0 rows affected (0.04 sec)
mysql> insert into mqs4 values('abc','abc'),('ABC','ABC');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from mqs4;
+------+------+
| col1 | col2 |
+------+------+
| abc | abc |
| ABC | ABC |
+------+------+
2 rows in set (0.00 sec)
mysql> select * from mqs4 where col1='abc';
+------+------+
| col1 | col2 |
+------+------+
| abc | abc |
| ABC | ABC |
+------+------+
2 rows in set (0.00 sec)
mysql> select * from mqs4 where col2='abc';
+------+------+
| col1 | col2 |
+------+------+
| abc | abc |
+------+------+
1 row in set (0.00 sec)
附1： 你是怎么确认所有的charset比较时，默认都不区分大小写？
可以看一下所有支持的charset以及其对应的默认 collation，就会发现都是"_ci"结尾的
mysql> show charset;
+----------+---------------------------------+---------------------+--------+
| 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 |
+----------+---------------------------------+---------------------+--------+
41 rows in set (0.00 sec)
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