This code work fine for me:

mysql> update link_productgroepen set groep_id=(select groep_id from link_productgroepen_bck  where link_productgroepen.sku=link_productgroepen_bck.sku); 

link_productgroepen_bck contains a backup of the table link_productgroepen ,so the structure is the same. In order to update the link_productgroepen table I need to drop it and to create a new empty clone of it that gets filled with the new values provided by an other website by an api. This new dataset needs to be complemented by information present in 2 columns in the link_productgroepen_bck table. The code above copies the contents of the groep_id column in the link_productgroepen_bck table to the renewed ink_productgroepen table if the sku value in both tables is the same. The other backupped column is copied by the same mysql command but with the other column name.