emqx4.3/plugins/emqttd_auth_mysql/etc/plugin.config

[
 {emysql, [
    {pool,      4},
    {host,      "localhost"},
    {port,      3306},
    {username,  "root"}, 
    {password,  "public"},
    {database,  "mqtt"},
    {encoding,  utf8}
 ]},
 {emqttd_auth_mysql, [
    {users_table, mqtt_users},
    {field_mapper, [
        {username, username},
        {password, password, plain}
    ]}
 ]}
].