schedule join multiple tables
You can have two fields in schedule table:


operation_type
operation_id


Then you can select everything like this:

SELECT s.*, l.weight, d.driver, d.fuel, mo.mechanic, mo.parts
  FROM schedule AS s
  LEFT JOIN loading_operation AS l ON s.operation_id=l.id
                                  AND s.operation_type='l'
  LEFT JOIN delivery_operation AS d ON s.operation_id=d.id
                                  AND s.operation_type='d'
  LEFT JOIN maintenance_operation AS mo ON s.operation_id=mo.id
                                  AND s.operation_type='m';


and the the output of this query would be something like:

id  date_from  date_to    operation_type operation_id weight driver fuel mechanic 
 1 2019-02-06 2019-03-14       m            5                               874  
 2 2019-01-16 2019-02-24       d            3                 431     838         
 3 2019-03-16 2019-03-03       m            5                               874  
 4 2018-12-16 2019-02-11       m            2                               975  
 5 2019-02-12 2019-03-03       d            1                 825     438         
 6 2019-02-12 2019-03-14       d            7                 604     534         
 7 2019-01-11 2019-02-25       l            9          304                       
 8 2019-03-15 2019-02-14       m            8                               188  
 9 2019-03-05 2007-04-20       m            2                               975  
10 2019-02-25 2007-04-22       m            5                               874