Select values from table having a field value same as another field value

I have the following columns in a table : FDA_ID and FDA_ID_PARENT and the following set of data:

FDA_ID  | FDA_ID_PARENT
 771    |   NULL
 772    |   771
 773    |   771
 774    |   0
 775    |   NULL
 776    |   775
 777    |   NULL

I have the FDA_ID = 773 as entry data and want to get those ‘linked’ to it.

What I mean by linked is those FDA_ID having the same FDA_ID as the FDA_ID_PARENT of FDA_ID = 773 and those having the same FDA_ID_PARENT as the FDA_ID_PARENT of FDA_ID = 773.

So in the case of FDA_ID = 773, the query would return FDA_ID = 771, 772 and 773.

I have tried the below query but it does not work:

select *
from FDA_TABLE FDA
where 
FDA.FDA_ID = 773
or 
FDA.FDA_ID in (select FDA_ID from FDA_TABLE where FDA_ID_PARENT = FDA.FDA_ID_PARENT);

Any help please?

Answer

The second part of your WHERE clause returns all rows that have a parent set.

FDA.FDA_ID in (select FDA_ID from FDA_TABLE where FDA_ID_PARENT = FDA.FDA_ID_PARENT)

You may want to use something like (or any parts of it):

SELECT
    *
FROM
    FDA_TABLE
WHERE
    -- self
    FDA_ID = 773
    -- parent
    OR FDA_ID = (
        SELECT FDA_ID_PARENT FROM FDA_TABLE WHERE FDA_ID = 773
    )
    -- siblings
    OR FDA_ID_PARENT = (
        SELECT FDA_ID_PARENT FROM FDA_TABLE WHERE FDA_ID = 773
    )
    -- children
    OR FDA_ID_PARENT = 773