Interconnecting tables on PostgreSQL - postgresql

Question

I am a newbie here.
I am using PostgreSQL to manipulate lots of data in my specific field of research. Unfortunately, I am encountering a problem that is not allowing me to continue my analysis. I tried to simplify my problem to clearly illustrate it.
Let's suppose I have a table called "Buyers" with those data:
table_buyers
The buyers can make ONLY ONE purchase in each store or none. There are three stores and there a table for each one. Just like below:
table_store1
table_store2
table_store3
To create the tables, I am using the following code:
CREATE TABLE public.buyer
(
ID integer NOT NULL PRIMARY KEY,
name text NOT NULL,
phone text NOT NULL
)
WITH (
OIDS = FALSE
)
;
CREATE TABLE public.Store1
(
ID_buyer integer NOT NULL PRIMARY KEY,
total_order numeric NOT NULL,
total_itens integer NOT NULL
)
WITH (
OIDS = FALSE
)
;
CREATE TABLE public.Store2
(
ID_buyer integer NOT NULL PRIMARY KEY,
total_order numeric NOT NULL,
total_itens integer NOT NULL
)
WITH (
OIDS = FALSE
)
;
CREATE TABLE public.Store3
(
ID_buyer integer NOT NULL PRIMARY KEY,
total_order numeric NOT NULL,
total_itens integer NOT NULL
)
WITH (
OIDS = FALSE
)
;
To add the information on the tables, I am using the following code:
INSERT INTO buyer (ID, name, phone) VALUES
(1, 'Alex', 88888888),
(2, 'Igor', 77777777),
(3, 'Mike', 66666666);
INSERT INTO Store1 (ID_buyer, total_order, total_itens) VALUES
(1, 87.45, 8),
(2, 14.00, 3),
(3, 12.40, 4);
INSERT INTO Store2 (ID_buyer, total_order, total_itens) VALUES
(1, 785.12, 7),
(2, 9874.21, 25);
INSERT INTO Store3 (ID_buyer, total_order, total_itens) VALUES
(2, 45.87, 1);
As all the tables are interconnected by buyer's ID, I wish I could have a query that generates an output just like this:
desired output table.
Please, note that if the buyer did not buy anything in a store, I must print '0'.
I know this is an easy task, but unfortunately, I have been failing on accomplish it.
Using the 'AND' logical operator, I tried the following code to accomplish this task:
SELECT
buyer.id,
buyer.name,
store1.total_order,
store2.total_order,
store3.total_order
FROM
public.buyer,
public.store1,
public.store2,
public.store3
WHERE
buyer.id = store1.id_buyer AND
buyer.id = store2.id_buyer AND
buyer.id = store3.id_buyer;
But, obviously, it just returned 'Igor' as this was the only buyer that have bought items on all three stores (print screen).
Then, I tried the 'OR' logical operator, just like the following code:
SELECT
buyer.id,
buyer.name,
store1.total_order,
store2.total_order,
store3.total_order
FROM
public.buyer,
public.store1,
public.store2,
public.store3
WHERE
buyer.id = store1.id_buyer OR
buyer.id = store2.id_buyer OR
buyer.id = store3.id_buyer;
But then, it returns 12 lines with wrong values (print screen).
Clearly, my mistake is about not considering that 'Buyers' don't have to on all three stores on my code. I just can't correct it on my own, can you please help me?
I appreciate a lot for an answer that can light up my way. Thanks a lot!
Tips about how I can search for this issue are very welcome as well!

Answer 1

Ok. I doubt that this is the final answer for you, but its a start
SELECT
buyer.id,
buyer.name,
COALESCE( gb_store1.total_orders, 0 ) as store1_total,
COALESCE( gb_store2.total_orders, 0 ) as store2_total,
COALESCE( gb_store3.total_orders, 0 ) as store3_total
FROM
public.buyer,
LEFT OUTER JOIN ( SELECT ID_buyer,
SUM( total_orders ) as total_orders,
SUM( total_itens ) as total_itens
FROM public.store1
GROUP BY ID_buyer ) gb_store1 ON gb_store1.id_buyer = buyer.id ,
LEFT OUTER JOIN ( SELECT ID_buyer,
SUM( total_orders ) as total_orders,
SUM( total_itens ) as total_itens
FROM public.store2
GROUP BY ID_buyer ) gb_store2 ON gb_store2.id_buyer = buyer.id ,
LEFT OUTER JOIN ( SELECT ID_buyer,
SUM( total_orders ) as total_orders,
SUM( total_itens ) as total_itens
FROM public.store3
GROUP BY ID_buyer ) gb_store3 ON gb_store3.id_buyer = buyer.id ;
So, this query has a couple elements should focus on. The subselects/groupby allow you to total within your subtables by ID_buyer. The LEFT OUTER JOIN make its so your query can still return a result, even if a subselect finds no matching record. Finally, the COALESCE allows you to return 0 when one of your totals is NULL (because the subselect found no match).
Hope this helps.