This SQL query selects all properties from the alf_node_properties table where the actual type is 21 and left joins the alf_content_data table to get content data by id, returning only rows where the content data id is null.

1. select * from alf_node_properties np left join alf_content_data cd on (cd.id = np.long_value) where
np.actual_type_n = 21 and cd.id is null;