fill table A with random ID's from another table B I have two tables and I want update the column GebruikerID from table Topic with the ID of the table Gebruiker with random values.

You must also know the follow things:


Topic.GebruikerID references to Gebruiker.ID.
Not every ID is used in both tables.
The random number that must be generated exist in the table Gebruiker (column ID) and must be updated into the table Topic (column GebruikerID).
I use SQL server in an express edition.


Here are some images.


The relation between the tables


The ID's of the Gebruiker-table
You can assign two ROW_NUMBERs based on a random sort and join on them:

select
   t.id,
   g.id as gebruikerid
from
 (
   select id,
      (row_number() over (order by newid())
    % (select count(*) from gebruiker))+1 as rn -- number from 1 to n 
   from topic
 ) as t 
join 
 ( select id,
     row_number() over (order by newid()) as rn -- sequential number from 1 to n
   from gebruiker
 ) as g
  on t.rn = g.rn 
order by t.id


See fiddle

EDIT:

You can use the result of this SELECT to update the Topic table:

update t
set gebruikerid = dt.gebruikerid
from topic as t
join
 (
   select
      t.id,
      g.id as gebruikerid
   from
    (
      select id,
         (row_number() over (order by newid()) % (select count(*) from gebruiker))+1 as rn
      from topic
    ) as t 
   join 
    ( select id,
         row_number() over (order by newid()) as rn
      from gebruiker
    ) as g
   on t.rn = g.rn 
 ) as dt
on t.id = dt.id;


fiddle