SQL left outer join multiple tables

Question

I have three tables and I think I need to create a left outer join.

Country:

country_id   country_name  
--------------------------
   15        United States  
   20        Sweden  

Partner:

partner_id    partner_name  
---------------------------
   1          Atlas Group  
   2          Jenkins  
   3          Roadmap  

Member:

member_id    name    registration date    partner_id    country_id  
------------------------------------------------------------------
   001       Bob     2014-01-02                1            15  
   002       Ken     2014-05-02                3            20  

I want to return a count of all members who have registered since 2014 by country name and partner name including all members who do not have a country name. (So this should include Ken with country_id of 200 which does not match to a country in the country table.

so far I have

SELECT 
    country.country_name, partner.partner_name 
FROM 
    COUNT (*) from member 
WHERE
    registration_date >= 2014-01-01

but I realize I am probably way off.

Thank you in advance for any assistance.

Answer 1

something like:

select 
    a.*
    ,p.partner_name
from
    (
        select 
            m.name
            ,c.country_name
            ,partner_id
        from 
            member as m 
            left outer join country as c
                on m.country_id = c.country_id
        where 
            registrationdate >= '2014-01-01'
    ) as a
    left outer join partner as p
        on p.partner_id = a.parner_id

when using two left outer joins you need wrap the first one in a sub query if you want to get matching rows from table 3 where there was no matching row in table 2.. if i am remembering correctly.

EDIT: sorry didnt see that count part of the question!

select 
    a.country
    ,p.partner_name
    ,count(*)
from
    (
        select 
            m.name
            ,c.country_name
            ,partner_id
        from 
            member as m 
            left outer join country as c
                on m.country_id = c.country_id
        where 
            registrationdate >= '2014-01-01'
    ) as a
    left outer join partner as p
        on p.partner_id = a.parner_id
group by 
    a.country
    ,p.partner_name

but watch out as all your nulls will be counted together.. so everyone with no country assigned is in the same country.

Answer 2

Unless I'm misunderstanding the question this is a simple LEFT JOIN of 2 tables.

SELECT country_name, partner_name, count(member_id)
FROM member
    LEFT JOIN partner USING (partner_id)
    LEFT JOIN country USING (country_id)
WHERE registration_date >= '2014-01-01'
GROUP BY partner_id, country_id

If you want to require the partner to exist then you may want to change LEFT JOIN partner ... to JOIN partner ... .

You could have used the ON syntax for the JOINs but then you would have to qualify the use of partner_id and country_id elsewhere in the query.