select sum(voorraad - aantal) AS 'nooit besteld' from artikelen inner join items ON artikelen.artikel_id=items.artikel_id;
+---------------+
| nooit besteld |
+---------------+
|           279 |
+---------------+
1 row in set (0.00 sec)

select sum(aantal) As januari from items inner join bestellingen ON items.bestelling_id=bestellingen.bestelling_id where datum between '2017-01-01' AND '2017-01-31';
+---------+
| januari |
+---------+
|      50 |
+---------+
1 row in set (0.00 sec)

select count(DISTINCT klanten.klant_id) - count(bestellingen.klant_id) As 'nog niets besteld' from klanten left outer join bestellingen ON klanten.klant_id=bestellingen.klant_id ;
+-------------------+
| nog niets besteld |
+-------------------+
|                20 |
+-------------------+
1 row in set (0.00 sec)

select count(klant_id) - (select count(klant_id) from bestellingen) As 'nog niets besteld' from klanten;
+-------------------+
| nog niets besteld |
+-------------------+
|                20 |
+-------------------+
1 row in set (0.00 sec)

select gemeente from bestellingen inner join klanten ON bestellingen.klant_id=klanten.klant_id group by gemeente order by count(bestellingen.klant_id) desc limit 3;
+----------+
| gemeente |
+----------+
| GENT     |
| MELLE    |
| LEMBERGE |
+----------+
3 rows in set (0.00 sec)