也许是这样的:

SELECT 
    id, 
    name, 
    COALESCE(trademark.name,design.name) AS object_name
FROM 
    opposition
    LEFT JOIN trademark 
        on opposition.object_id =   trademark.id
        AND trademark.object_table ='trademark'
    LEFT JOIN design
        on opposition.object_id =   design.id
        AND design.object_table ='design'