Question

Say

Table A             Table AB         Table B
+----+---------+    +-----+-----+    +----+-----+
| id | name    |    | ida | idb |    | id | age |
+----+---------+    +-----+-----+    +----+-----+
|  1 |   'one' |    |   1 |   3 |    |  3 |   3 |
+----+---------+    +-----+-----+    +----+-----+
|  2 |   'two' |    |   2 |   4 |    |  5 |   5 |
+----+---------+    +-----+-----+    +----+-----+
|  3 | 'three' |    |   2 |   5 |
+----+---------+    +-----+-----+

What I want           What I get
+---------+------+    +---------+------+
| name    | age  |    | name    | age  |
+---------+------+    +---------+------+
|   'one' |    3 |    |   'one' |    3 |
+---------+------+    +---------+------+
|   'two' |    5 |    |   'two' |    5 |
+---------+------+    +---------+------+
| 'three' | NULL |    |   'two' | NULL |
+---------+------+    +---------+------+
                      | 'three' | NULL |
                      +---------+------+

My sql is

SELECT A.name, B.age
FROM A
LEFT JOIN AB
  ON A.id = AB.ida
LEFT JOIN B
  ON AB.idb = B.id

Constraints: I don’t want to use Where statement for specific reasons, nor sub-queries for performance reason.

Is there a way I could only get the records where the relation AB matches something that exists only using JOIN/ON and no sub-queries?

Answer 1

Use grouping.

SELECT A.name, Max(B.age)
FROM A 
LEFT JOIN AB 
  ON A.id = AB.ida 
LEFT JOIN B 
  ON AB.idb = B.id 
GROUP BY a.id, a.Name 
order by a.id

Or you could use right joins.

select a.name, b.age
from 
    b 
    inner join ab on b.id = ab.idb
    right join a on ab.ida = a.id