char、varchar、binary和varbinary的区别与联系

char使用固定长度的空间进行存储，char(4)存储4个字符，根据编码方式的不同占用不同的字节，gbk编码方式，不论是中文还是英文，每个字符占用2个字节的空间，utf8编码方式，每个字符占用3个字节的空间。 如果需要存储的字符串的长度跟所有值的平均长度相差不大，适合用char，如MD5。 对于经常改变的值，char优于varchar，原因是固定长度的行不容易产生碎片。 对于很短的列，char优于varchar，原因是varchar需要额外一个或两个字节存储字符串的长度。 varchar保存可变长度的字符串，使用额外的一个或两个字节存储字符串长度，varchar(10),除了需要存储10个字符，还需要1个字节存储长度信息(10),超过255的长度需要2个字节来存储 例外：Myisam引擎中使用ROW_FORMAT=FIXED时，每行使用相同的空间，造成浪费 char和varchar后面如果有空格，char会自动去掉空格后存储，varchar虽然不会去掉空格，但在进行字符串比较时，会去掉空格进行比较

+-------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(4) | YES | | NULL | | | addr | char(8) | YES | | NULL | | | bn | varbinary(4) | YES | | NULL | | | b | binary(8) | YES | | NULL | | +-------+--------------+------+-----+---------+----------------+ +----------------------+----------------------+ | concat("",addr,"asdf | a | | asdf | a | | a | a | | a | a | | t a | a | +----------------------+----------------------+ mysql> select * from zcy where name='a '; //由于name是varchar，比较时，'a '自动转换为'a'

+----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 3 | a | a | ab | ab | | 4 | a | a | ab | a | +----+------+------+------+----------+ 2 rows in set (0.00 sec) mysql> select * from zcy where name='a'; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 3 | a | a | ab | ab | | 4 | a | a | ab | a | +----+------+------+------+----------+ 2 rows in set (0.00 sec) +-------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(4) | YES | | NULL | | | addr | char(8) | YES | | NULL | | | bn | varbinary(4) | YES | | NULL | | | b | binary(8) | YES | | NULL | | +-------+--------------+------+-----+---------+----------------+ +--------------------+-------------------+ | concat("",b,"ab a | NULL | | ab | ab | | ab | ab | | ab | a | | NULL | a | | NULL | abcde | | NULL | abcd1234 | +--------------------+-------------------+ binary保存二进制字符串，它保存的是字节而不是字符，没有字符集限制 binary(8)可以保存8个字符，每个字符占1个字节，共占8个字节 进行比较时是按字节进行比较，而不是按字符（char），按字节比较比字符简单快速 按字符比较不区分大小写，而binary区分大小写，结尾使用\0填充，而不是空格 mysql> select * from zcy where b='a\0\0\0\0\0\0\0'; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 5 | t a | a | NULL | a | +----+------+------+------+----------+ mysql> select * from zcy where b='a \0\0\0\0\0\0'; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 4 | a | a | ab | a | +----+------+------+------+----------+ varbinary保存变长的字符串，后面不会补\0 mysql> select * from zcy where bn='ab'; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 3 | a | a | ab | ab | +----+------+------+------+----------+ 1 row in set (0.01 sec) mysql> select * from zcy where bn='ab '; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 2 | asdf | a | ab | ab | +----+------+------+------+----------+ 1 row in set (0.00 sec) mysql> select * from zcy where bn='ab '; +----+------+------+------+----------+ | id | name | addr | bn | b | +----+------+------+------+----------+ | 4 | a | a | ab | a | +----+------+------+------+----------+ 1 row in set (0.00 sec)