从具有嵌套关系的一张表中获取数据

Question

I am new in DB and I have a table topics and in this table, I have a foreign key master_topic_id and this foreign key is related to the same table topics column id. Schema:

CREATE TABLE public.topics (
id bigserial NOT NULL,
created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
published_at timestamp NULL,
master_topic_id int8 NULL,
CONSTRAINT t_pkey PRIMARY KEY (id),
CONSTRAINT t_master_topic_id_fkey FOREIGN KEY (master_topic_id) REFERENCES topics(id
);

I write a query - SELECT * FROM topics WHERE id = 10. But if this record has master_topic_id I need to get data by master_topic_id too. I tried to do it by using JOIN, but join just concat records, but I need to have data from master_topic_id as new row. Any help?

Answer 1

I think you are describing:

select t.*
from topics t
where t.id = 10 or
      exists (select 1
              from topics t2
              where t2.master_topic_id = t.id and t2.id = 10
             );

However, you might just want:

where 10 in (id, master_topic_id)

Answer 2

Use or in your where condition

SELECT * 
FROM topics 
WHERE id = 10 
or master_topic_id = 10

you can use union all as well

SELECT * 
FROM topics 
WHERE id = 10 

union all

SELECT * 
FROM topics 
WHERE master_topic_id = 10