CHAR
char (M) M字符，长度是M*字符编码长度，M最大255。
验证如下：
mysql> create table t1(name char(256)) default charset=utf8;
ERROR 1074 (42000): Column length too big for column 'name' (max = 255); use BLOB or TEXT instead
mysql> create table t1(name char(255)) default charset=utf8;
Query OK, 0 rows affected (0.06 s
