Postgresの複数の関連テーブルからデータを取得するにはどうすればよいですか？

SELECT ROW_TO_JSON(T)
FROM    (
        SELECT t1.id,
               t1.name,
               array_to_json(array((
                 select case when t2.id is not null then row_to_json(t2) 
                             when t3.id is not null then row_to_json(t3) 
                             when t4.id is not null then row_to_json(t4) 
                        end
                 from   public.relation r 
                 LEFT JOIN public.table1 t2 on r.table2 = t2.id
                 LEFT JOIN public.table1 t3 on r.table3 = t3.id
                 LEFT JOIN public.table1 t4 on r.table4 = t4.id
                 where  r.table1 = t1.id
               ))) related_items
        FROM   public.table1 t1
       ) T