MySQL数据类型--------字符串类型实战

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_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)

　 * 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。