How to join these queries?
In advance i've never written anything in mysql, but the theory should still be there in case of small syntax errors.

In general i'd advice using INNER JOINS, instead of joining your tables in a WHERE clause, due to easier readability and better coding practice, as it makes it easy to change the sql inner join to an outer join as i've shown below:

SELECT v.Voorstellingsnummer, COUNT(r.Voorstellingsnummer)+1 
FROM voorstelling v
INNER JOIN reserveringen r ON r.Voorstellingsnummer = v.Voorstellingsnummer
WHERE v.Vestigingsnaam = 'Amsterdam' 
AND v.Filmnaam = 'Avengers' 
GROUP BY r.Voorstellingsnummer HAVING COUNT(*) &lt; 50


I am however very confused as to how you want the counts added? The above query should return what you're asking for. As it sounds like you just want the initial counts to be added with 1. To have this match more function like instead of a hardcoded value, i would have to understand why 4 and 5 should have a count of 1 in the other table?