指定密钥太长 - 最大密钥长度为767字节

Question

当我执行以下命令时:

create table assessment (
    id integer not null auto_increment unique,
    assignment_weight_type tinyint not null,
    description varchar(255),
    end_date datetime not null,
    from_grade tinyint not null,
    f_id tinyint not null,
    name varchar(255) not null,
    start_date datetime not null,
    status tinyint not null,
    weight smallint,
    school_id integer not null,
    school_year_id integer not null,
    s_id integer, primary key (id),
    unique (name, school_id, school_year_id, from_grade, f_id)
) ENGINE=InnoDB;

我收到此错误消息:

Specified key was too long; max key length is 767 bytes

我正在使用utf8mb4的字符集.那我为什么收到错误信息呢？

Answer 1

utf8mb4 每个字符最多使用四个字节,因此名称本身可以占用多达1020个字节.