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
|
1
2
3
4
5
6
7
|
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 ]
|
1
2
3
4
5
6
7
|
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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
|
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
|
1
2
3
4
5
6
7
8
9
10
|
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。
本文转自asd1123509133 51CTO博客，原文链接：http://blog.51cto.com/lisea/1943246，如需转载请自行联系原作者
