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}
   ]}
]}