Question

I have two tables:

id
1
2
3
4

t1 AND t2

id | related_id
1  | 2
1  | 3

Where t2 is relationship table between t1 records. What is the best way to get desired output?

t1.id | t1_copy.id
1     | NULL -- want to get this NULL row
1     | 2
1     | 3

Simple JOIN would almost work, however it doesn’t give me the first NULL row.

SELECT t1.id, t1_copy.id FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t1 t1_copy ON t1_copy.id = t2.related_id
WHERE t1.id = 1

P.S: Yes, I do realize that desired output is wacky.

Answer 1

Seems like a simple UNION should do the trick

SELECT
   id, 
   null as copy_id
FROM
   t1 
WHERE
  exists (select * from t2 where t1.id = t2.id)
UNION ALL
SELECT
   t1.id,
   t2.related_id
FROM
   t1 
   INNER JOIN t2
   ON t1.id = t2.id

SQL Fiddle