title: Average problems with different tables
body: <p>I have a database with the following structure with the following registers</p>
  
  <pre><code>CREATE TABLE  aircraft  (
     number_aircraft  int(11) ,
     type_aircraft  VARCHAR(10),
    PRIMARY KEY ( number_aircraft )
  );
  
  
  CREATE TABLE  passenger  (
     number_flight  int(11) ,
     ID  int(11),
     Name  char(10) DEFAULT NULL,
    PRIMARY KEY ( number_flight , ID ),
    FOREIGN KEY ( number_flight ) REFERENCES  flight  ( number_flight )
  );
  
  
  CREATE TABLE  flight  (
     number_flight  int(11) ,
     since  char(2) ,
     until  char(2) ,
     Date_flight  date ,
     number_aircraft  int(11),
    PRIMARY KEY ( number_flight,number_aircraft ),
    FOREIGN KEY ( number_aircraft ) REFERENCES  aircraft  ( number_aircraft )
  );
  
  
  
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (1,'B-777');
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (2,'B-755');
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (3,'B-775');
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (4,'B-505');
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (5,'Airbus A3');
  INSERT INTO  aircraft  ( number_aircraft , type_aircraft ) VALUES (6,'B-707');
  
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (1,34889876,'German');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (1,44859776,'Ariel');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (2,34587880,'Andres');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (2,34886463,'David');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (3,24757385,'Juan');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (3,34885768,'Marcos');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (4,10000001,'Bárbara');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (4,20000002,'Daniela');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (5,20000003,'Alejandra');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (5,20000004,'Melina');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (6,30000001,'Daniela');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (6,30000002,'Valeria');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (7,40000001,'Silvana');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (7,50000001,'Cecilia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (8,50000002,'Noelia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (8,50000003,'Cintia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (9,60000001,'Mariela');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (9,60000002,'Ornella');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (10,60000003,'Cecilia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (10,60000004,'Valeria');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (11,60000005,'Agustina');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (11,60000006,'Aldana');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (12,60000007,'Carolina');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (12,60000008,'Cecilia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (13,70000001,'Bárbara');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (13,70000002,'Gisele');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (14,70000003,'Cecilia');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (14,70000004,'Vicuntilria');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (15,60000009,'Sabrina');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (15,60000010,'Agostina');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (16,80000001,'Andrea');
  INSERT INTO  passenger  ( number_flight , ID , Name ) VALUES (16,80000002,'Tamara');
  
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (1,'A','A','2001-02-14',1);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (2,'A','F','2010-05-15',1);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (3,'A','D','2012-08-09',4);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (4,'B','H','2001-02-14',3);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (5,'D','H','2005-06-17',4);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (6,'A','D','2007-03-17',1);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (7,'A','F','2001-02-19',2);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (8,'C','F','2011-03-14',2);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (9,'A','B','2013-03-02',4);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (10,'A','B','2010-03-12',3);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (11,'A','D','2011-04-28',3);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (12,'C','B','2013-01-01',4);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (13,'B','D','2013-03-21',5);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (14,'A','B','2012-03-02',5);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (15,'D','H','2013-06-06',6);
  INSERT INTO  flight  ( number_flight , since , Until , Date_flight , number_aircraft ) VALUES (16,'A','H','2013-12-13',6);
  </code></pre>
  
  <p>I have to list the average number of passengers transported by aircraft
  company, by aircraft type, as I do? attempt to use the avg () but
  I do not use it when I have a join.</p>