1. 背景
* MySQL支持的字符串类型有CHAR、VARCHAR、二进制、var二进制、BLOB、文本等等。
2. 字符串类型所需的存储和值范围
| 类型 | 说明 | N的含义 | 是否有字符集 | 最大长度 |
| CHAR(N) | 定义字符 | 字符 | 是 | 255 |
| VARCHAR(N) | 变长字符 | 字符 | 是 | 16384 |
| BINARY(N) | 定长二进制字节 | 字节 | 否 | 255 |
| VARBINARY(N) | 变长二进制字节 | 字节 | 否 | 16384 |
| TINYBLOB | 二进制大对象 | 字节 | 否 | 256 |
| BLOB | 二进制大对象 | 字节 | 否 | 16K |
| MEDIUMBLOB | 二进制大对象 | 字节 | 否 | 16M |
| LONGBLOB | 二进制大对象 | 字节 | 否 | 4G |
| TINYTEXT | 大对象 | 字节 | 是 | 256 |
| TEXT | 大对象 | 字节 | 是 | 16K |
| MEDUIMBLOB | 大对象 | 字节 | 是 | 16M |
| LONGTEXT | 大对象 | 字节 | 是 | 4G |
3. 定义与变长区别 (CHAR VS VARCHAR)
| 值 | CHAR(4) | 占用空间 | VARHCAR(4) | 占用空间 |
| '' | ' ' | 4 bytes | '' | 1 bytes |
| 'ab' | 'ab ' | 4 bytes | 'ab' | 3 bytes |
| 'abcd' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
| 'abcdefgh' | 'abcd' | 4 bytes | 'abcd' | 5 bytes |
4. 字符串类型相关注意事项
* 在BLOB和TEXT列上创建索引时，必须制定索引前缀的长度
* VARCHAR和VARBINARY必须长度是可选的
* BLOB和TEXT列不能有默认值
* BLOB和TEXT列排序时只使用该列的前max_sort_length个字节
mysql> show variables like 'max_sort_length';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_sort_length | 1024 |
+-----------------+-------+
1 row in set (0.01 sec)5. 字符串类型与排序规则
* 查看字符串相关信息 [ Default collation: 默认排序规则 ]
mysql> show character set like 'utf8mb4';
+---------+---------------+--------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+---------------+--------------------+--------+
| utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 |
+---------+---------------+--------------------+--------+
1 row in set (0.01 sec)* 查看utfmb4所有排序规则
mysql> show collation like '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_spanish3_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)* 修改默认排序规则
mysql> set names utf8mb4 collate utf8mb4_bin;
Query OK, 0 rows affected (0.00 sec)
mysql> show character set like 'utf8mb4';
+---------+---------------+--------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+---------------+--------------------+--------+
| utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 |
+---------+---------------+--------------------+--------+
1 row in set (0.00 sec)6. 总结
以需求驱动技术，技术本身没有优略之分，只有业务之分。
