SQL Server : left outer join on two relations to 2 tables - sql-server

Question

I have 2 tables with primary keys and third or many table which references these 2 primary tables and have some extra values on one or both primary keys.
I need to create some SQL which will always deliver result with as much information as possible by joining these 3 tables. Best result - all 3 tables joined. Medium result - at least some primary keys (or both) are selected. Worst result all columns are null.
Main idea is to have combination of two primary tables and many extra tables which could be empty but should allow results from tables with values.
I tried to start with 3 tables but got stuck on second join.
It works for me only when I join first table. Joining second one produces error.
What should I use instead of ? as SQL statement?
http://sqlfiddle.com/#!18/7438b/3
CREATE TABLE [AGENCIES]
(
[AGENCY_NAME] [CHAR](9),
id INT IDENTITY(1,1) NOT NULL PRIMARY KEY
);
CREATE TABLE [PERSONS]
(
[NAME] [CHAR](9),
id INT IDENTITY(1,1) NOT NULL PRIMARY KEY
);
CREATE TABLE [AGENCY_PERSON]
(
agency_id INT FOREIGN KEY REFERENCES agencies(id),
person_id INT FOREIGN KEY REFERENCES persons(id),
[TITLE] [CHAR](9) NULL,
id INT IDENTITY(1,1) NOT NULL PRIMARY KEY
);
INSERT INTO agencies (AGENCY_NAME)
VALUES ('AgencyOne'), ('AgencyTwo'), ('Agency3');
INSERT INTO persons (name)
VALUES ('PersonOne'), ('PersonTwo'), ('Person3');
INSERT INTO AGENCY_PERSON (agency_id, person_id, title)
VALUES (1, 1, 'TitleOne'), (1, 2, 'TitleTwo');
SELECT * FROM AGENCY_PERSON;
-- works fine for one primary table
SELECT [AGENCY_NAME], [TITLE]
FROM agencies
LEFT OUTER JOIN [AGENCY_PERSON] ON [AGENCY_PERSON].agency_id = agencies.id
WHERE [AGENCY_NAME] = 'AgencyOne';
-- error for two primary tables: Msg 4104 - The multi-part identifier "agencies.id" could not be bound.
SELECT [AGENCY_NAME], [TITLE], persons.name
FROM agencies, persons
LEFT OUTER JOIN [AGENCY_PERSON] ON [AGENCY_PERSON].agency_id = agencies.id
AND [AGENCY_PERSON].person_id = persons.id
WHERE [AGENCY_NAME] = 'AgencyOne';
-- select ? 'AgencyOne' - all records exist
-- AgencyOne, TitleOne, PersonOne
-- select ? 'TitleTwo' - both records on primary tables exist, but no in join table
-- AgencyOne, TitleTwo, NULL
-- select ? 'Agency3' - one of primary tables exist
-- Agency3, NULL, NULL
-- select ? 'Title3' - one of primary tables exist
-- NULL, Title3, NULL
-- select ? 'AgencyX' - nothing exists
-- NULL, NULL, NULL
forpas gave good answer but it is in reverse. Extra tables are left joined by primary which requires extra tables exist and have values. What I need is opposite - extra tables should join primaries. For example it could be more extra tables like PERSON_PHONE, PERSON_ADDRES or AGENCY_PERSON_LOCATION. As soon as agency or person exist (but no values in these extra tables) result should be row with existing agency and person and nulls in all other columns from the joined tables.

Answer 1

Your code would work if you did not use that old style (cross) join:
from agencies, persons
So write it like this:
select a.[AGENCY_NAME], ap.[TITLE], p.name
from agencies as a cross join persons as p
left outer join [AGENCY_PERSON] as ap
on ap.agency_id = a.id and ap.person_id = p.id
where a.[AGENCY_NAME] = 'AgencyOne';
I used aliases for all the tables involved and I qualified all the columns with the aliases of the tables they belong.
Results:
> AGENCY_NAME | TITLE | name
> :---------- | :-------- | :--------
> AgencyOne | TitleOne | PersonOne
> AgencyOne | TitleTwo | PersonTwo
> AgencyOne | null | Person3
I'm not sure if this is what you want as output but I believe you see now how you can join all 3 tables.
In case you want only the matching rows of the tables, then you should do inner joins:
select a.[AGENCY_NAME], ap.[TITLE], p.name
from [AGENCY_PERSON] as ap
inner join agencies as a on ap.agency_id = a.id
inner join persons as p on ap.person_id = p.id
where a.[AGENCY_NAME] = 'AgencyOne';
Results:
> AGENCY_NAME | TITLE | name
> :---------- | :-------- | :--------
> AgencyOne | TitleOne | PersonOne
> AgencyOne | TitleTwo | PersonTwo
See the demo.