how to do special join on table A and B (keep A row and add B columns only when there is one on one match)

Question

I want to add to table A informations in table B only for the rows of table A having one and only one row match in table B.

Example : GIVEN table A

+ lastName firstName
+ A        B
+ C        D
+ E        F

AND table B

* lastName firstName age
* A        B         10
* C        D         15
* C        D         20

I WANT TABLE RESULT

* lastName firstName age
* A        B         10
* C        D         null
* E        F         null

Is it possible to do it in SQL? left join is not sufficient

Answer 1

Kind of

select A.lastName, A.firstName, u.age
from A
left join (
   select lastName, firstName, max(age) age
   from B
   group by lastName, firstName
   having count(*) = 1 -- or may be count(distinct age) = 1
) u on u.lastName = A.lastName and u.firstName = A.firstName

Answer 2

I think you want to add unique rows in B to A . For this, I would suggest UNION ALL :

select firstname, lastname, max(age) as age
from b
group by firstname, lastname
having count(*) = 1
union all
select firstname, lastname, null as age
from a
where not exists (select 1
                  from from b
                  group by firstname, lastname
                  having count(*) = 1
                 );

This should, at least, produce the results in your question.