Speeding up left join query

Question

I have this query which gets me some data from a few tables. The query works perfectly, but it takes the server a long time to get the results.

$query_serial       = "SELECT serienummers.serienummer, salesregels.aantalartikelen FROM serienummers
                            LEFT JOIN 
                            (
                                SELECT aantalartikelen, artikelcode, salescode FROM salesregels
                            )salesregels ON salesregels.salescode = serienummers.salescode
                            LEFT JOIN 
                            (
                                SELECT factuurnummer, code FROM sales
                            )sales ON sales.code = serienummers.salescode                            
                            WHERE serienummer = '$serialnumber' AND salesregels.artikelcode = serienummers.artikelcode AND sales.factuurnummer != '-1'
                            ";

I've read a few thing online about indexing the tables, but im not quite sure how to apply this. Could anyone point me in the right direction, please?

My tables:

Serienummers:
+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| code          | int(11)      | NO   | PRI | 0       |       |
| serienummer   | varchar(100) | NO   |     |         |       |
| artikelcode   | int(11)      | YES  |     | 0       |       |
| datum         | int(11)      | NO   |     | 0       |       |
| usercode      | int(11)      | NO   |     | 0       |       |
| salescode     | int(11)      | YES  |     | 0       |       |
| reparatiecode | int(11)      | YES  |     | 0       |       |
| actie         | varchar(250) | NO   |     |         |       |
| rmacode       | int(11)      | NO   |     | 0       |       |
+---------------+--------------+------+-----+---------+-------+
Salesregels
+---------------------------+-------------------------------------------------+-
-----+-----+-----------+-------+
| Field                     | Type                                            |
Null | Key | Default   | Extra |
+---------------------------+-------------------------------------------------+-
-----+-----+-----------+-------+
| code                      | int(11)                                         |
NO   | PRI | 0         |       |
| salescode                 | int(11)                                         |
NO   |     | 0         |       |
| aantalartikelen           | double                                          |
NO   |     | 0         |       |
| artikelcode               | int(11)                                         |
NO   |     | 0         |       |
| prijsperstukinclbtw_old   | varchar(20)                                     |
NO   |     | 0         |       |
| btwtypecode               | int(11)                                         |
NO   |     | 0         |       |
| artikeltekst              | varchar(250)                                    |
NO   |     |           |       |
| procentbtw_old            | varchar(10)                                     |
NO   |     | 0,00      |       |
| isprijsonderdrukt         | int(11)                                         |
NO   |     | 0         |       |
| volgordenummer            | int(11)                                         |
NO   |     | 0         |       |
| prijsperstukinclbtw       | double                                          |
NO   |     | 0         |       |
| procentbtw                | double                                          |
NO   |     | 0         |       |
| korting                   | double                                          |
NO   |     | 0         |       |
| kortingtype               | enum('procenten','eurosinclbtw','eurosexclbtw') |
NO   |     | procenten |       |
| inkoopprijsperstukexclbtw | double                                          |
NO   |     | 0         |       |
| inkoopprijsprocentbtw     | double                                          |
NO   |     | 0         |       |
| inkoopprijsbtwtypecode    | int(11)                                         |
NO   |     | 0         |       |
| dagengarantie             | int(11)                                         |
NO   |     | 0         |       |
+---------------------------+-------------------------------------------------+-
Sales
+-------------------------+--------------+------+-----+---------+-------+
| Field                   | Type         | Null | Key | Default | Extra |
+-------------------------+--------------+------+-----+---------+-------+
| code                    | int(11)      | NO   | PRI | 0       |       |
| salestypecode           | int(11)      | NO   |     | 0       |       |
| websitesalesidcode      | int(11)      | NO   |     | 0       |       |
| datum                   | int(11)      | NO   |     | 0       |       |
| datumbetaald            | int(11)      | NO   |     | 0       |       |
| datumlaatstgewijzigd    | int(11)      | NO   |     | 0       |       |
| usercodelaatstgewijzigd | int(11)      | NO   |     | 0       |       |
| adrescode               | int(11)      | NO   |     | 0       |       |
| usercode                | int(11)      | NO   |     | 0       |       |
| bedragvoldaan1_old      | varchar(15)  | NO   |     | 0       |       |
| bedragvoldaan1          | double       | NO   |     | 0       |       |
| betalingswijzecode1     | int(11)      | NO   |     | 0       |       |
| bedragvoldaan2_old      | varchar(15)  | NO   |     | 0       |       |
| bedragvoldaan2          | double       | NO   |     | 0       |       |
| betalingswijzecode2     | int(11)      | NO   |     | 0       |       |
| opmerkingen             | longtext     | NO   |     | NULL    |       |
| opmerkingenextern       | longtext     | YES  |     | NULL    |       |
| voorraadlokatiecode     | int(11)      | NO   |     | 0       |       |
| computernaam            | varchar(255) | YES  |     | NULL    |       |
| factuurnummer           | bigint(20)   | NO   |     | 0       |       |
| betaald                 | int(11)      | NO   |     | 1       |       |
| status                  | varchar(255) | YES  |     | NULL    |       |
| dagenbetalingstermijn   | int(11)      | NO   |     | 8       |       |
| externordernummer       | varchar(255) | YES  |     | NULL    |       |
| externfactuurnummer     | varchar(255) | YES  |     | NULL    |       |
| isdeleted               | int(11)      | NO   |     | 0       |       |
+-------------------------+--------------+------+-----+---------+-------+

Answer 1

You do not need selecting everything using inner query before doing the join. The joins could be done directly.

select
sr.serienummer,
sg.aantalartikelen
from serienummers sr
left join salesregels sg on sg.salescode = sr.salescode and sg.artikelcode = sr.artikelcode
left join sales s on s.code = sr.salescode
where 
sr.serienummer = '$serialnumber'
and s.factuurnummer != '-1' ;

Now to speed up you need indexes on the joining keys and keys used in where clause.

alter table serienummers add index sales_kel_code(salescode,artikelcode);
alter table serienummers add index serienummer_idx(serienummer);
alter table salesregels add index salescode_idx(salescode);
alter table sales add index factuurnummer_idx(factuurnummer);