Question

1) I have two table:

+------                      +---------
topic                         mytask
+---------                   +---------
ID | What                    ID | TopicID
----------                   ------------
1  | A                        1 | 1
2  | B

2) I have SQL as below:

SELECT 
  a.id, b.id
FROM 
  topic a
JOIN (
 SELECT 
  b.id,b.topicid
 FROM 
  mytask b
) b on (b.topicid=a.id)

3) I have output as below (1 ROW ONLY):

ID | ID
-------
1  | 1

My expected output is as below (2 ROW, PRIORITY TO TOPIC BY LISTING THEM ALL) :

a.ID | b.ID
-------------
1    | MATCHED - OK - TAKE IT!!!
2    | NULL or what,ever...

How can i do that?

Answer 1

Use a left outer join:

SELECT 
  a.id, b.id
FROM 
  topic a
LEFT JOIN (
 SELECT 
  b.id,b.topicid
 FROM 
  mytask b
) b on (b.topicid=a.id)

and simplify the query to

SELECT 
  a.id, b.id
FROM 
  topic a
LEFT JOIN 
  mytask b ON b.topicid = a.id