I've got a MySQL problem

i wish to have caused by the following query right into a var and employ it to update in to the trigger i'm looking to get what they are called comma separated

The query returns more the other row

SELECT t.naam
FROM    trefwoorden t
INNER JOIN organisaties_has_trefwoorden AS o ON (t.id_trefwoorden = o.id_trefwoorden)
WHERE   o.id_organisaties = NEW.id_organisaties;

This is actually the trigger

CREATE TRIGGER updCheck_After AFTER
UPDATE ON organisaties_has_trefwoorden 
FOR EACH row    

    UPDATE organisaties o
    SET    o.trefwoorden_flat = 
    (
        SELECT t.naam
        FROM    trefwoorden t
        INNER JOIN organisaties_has_trefwoorden AS o ON (t.id_trefwoorden = o.id_trefwoorden)
        WHERE   o.id_organisaties = NEW.id_organisaties;        
    )
    WHERE  o.id_organisaties = NEW.id_organisaties  

The question can you really choose more rows as 1 column comma separated or it is possible to other solution ?

someone an indicator ?

UPDATE  organisaties o
SET     o.trefwoorden_flat = 
        (
        SELECT  GROUP_CONCAT(t.naam SEPARATOR ',')
        FROM    trefwoorden t
        INNER JOIN
                organisaties_has_trefwoorden AS o
        ON      t.id_trefwoorden = o.id_trefwoorden
        WHERE   o.id_organisaties = NEW.id_organisaties       
        )
WHERE   o.id_organisaties = NEW.id_organisaties