Question

Based on the below tables

Table_A

Id  RelatedId
---------------
1   1
1   2
1   3
2   4
2   5
2   2
3   7
3   8
4   9

Table_B

RelatedId  Name
--------------
2          A
3          B

I want to get all the rows from Table_A which have at least one matching relatedid in Table_B. The Ids from Table_A that do not have match in Table_B will have single row in the result table.

So output (result table) in this case will be

Id  RelatedId
---------------
1   1
1   2
1   3
2   4
2   5
2   2
3   Null
4   Null

EDIT: Seems like the question text is confusing for many. So a detailed explanation:

Table_A Id 1 has both 2 and 3 (Related Ids) matching in table_B. So output will have all the rows for 1 from Table_A. Similary Id 2 from Table_A has 2 (Related Id) matching in table_B. So all rows corresponding to 2 from Table_A will be picked up. Since 3 does not have any matching relatedid in table_B it will be displayed but with NULL as relatedid in the results table.

Answer 1

with validids as (
 select distinct id 
 from Table_A 
  inner join Table_B on 
   Table_A.relatedid = Table_B.relatedid
)
select 
  id,
  relatedid
from Table_A 
where id in (select id from validids)
union 
select distinct
 id, 
 null
from Table_A
where id not in (select id from validids)