PostgreSQL Inner Join on the same table + second table? - sql

Question

If this is a stupid question, forgive me, I'm not very familiar with PostgreSQL.
I've collected inventory data from used car dealerships in my area and stored it in a postgreSQL table. I've got a second table with particular details regarding certain makes and models. For example:
The dealership table is structured like so:
-----------------------------------------
| Dealership | Make | Model | Year | ID |
----------------------------------------|
| A | Ford | F250 | 2003 | 1 |
| A | Chevy| Cobalt| 2005 | 2 |
| B | Ford | F250 | 2003 | 1 |
| B | Dodge| Chrgr | 2012 | 3 |
-----------------------------------------
The details table is structured like so:
-----------------------------------------
| ID | DetailA| DetailB| DetailC|
-----------------------------------------
| 1 | data | data | data |
| 2 | data | data | data |
| 3 | data | data | data |
| 4 | data | data | data |
-----------------------------------------
My goal is to retrieve vehicle matches from multiple dealerships and display the appropriate details. In the above example, I would like to see:
-----------------------------------------------------
| Make | Model | Year | DetailA | DetailB | DetailC |
-----------------------------------------------------
| Ford | F250 | 2003 | data | data | data |
-----------------------------------------------------
With this result, I will know that both A and B havea 2003 Ford F250 for sale, and can view the related details of the vehicle.
I've tried many different queries, but most are variations on something like this:
SELECT DISTINCT
dealership_table.make,
dealership_table.model,
dealership_table.year
details_table.detaila,
details_table.detailb,
details_table.detailc
FROM
dealership_table
INNER JOIN
details_table
ON
dealership_table.id = details_table.id
WHERE
dealership_table.dealership = 'A'
OR
dealership_table.dealership = 'B'
However this returns all of the distinct matches from the table where dealership is either A or B. I've tried multiple inner-joins, but I an error complaining details_table is specified multiple times.
If I'm doing something really silly, I apologize. Like I said before, I'm pretty much an SQL noob.
What am I doing wrong? How should I go about retrieving the desired results? Any suggestions, solutions, or advice is greatly appreciated!

Answer 1

You can write:
SELECT dealership_table1.make,
dealership_table1.model,
dealership_table1.year,
details_table.detaila,
details_table.detailb,
details_table.detailc
FROM dealership_table dealership_table1
JOIN dealership_table dealership_table2
ON dealership_table1.make = dealership_table2.make
AND dealership_table1.model = dealership_table2.model
AND dealership_table1.year = dealership_table2.year
JOIN details_table
ON dealership_table.id = details_table.id
WHERE dealership_table1.dealership = 'A'
AND dealership_table1.dealership = 'B'
;
(Note that the FROM dealership_table dealership_table1 and JOIN dealership_table dealership_table2 set up distinct "aliases", so you can use the same table multiple different times in the same query without getting name-conflicts.)

Answer 2

I may be misunderstanding your table layout, but I think you should consider changing to a different structure. Here's what I would propose:
Vehicle:
----------------------------
| ID | Make | Model | Year |
----------------------------
| 1 | Ford | F250 | 2003 |
| 2 | Chevy| Cobalt| 2005 |
| 3 | Dodge| Chrgr | 2012 |
----------------------------
Dealership:
----------------------------
| Dealership | ID | Detail |
----------------------------
| A | 1 | data |
| A | 2 | data |
| B | 1 | data |
| B | 3 | data |
----------------------------
This way you're not storing vehicle information (make/model/year) in more than one place.
Here's how you would write your desired query given the above schema:
SELECT Make, Model, Year, A.Detail, B.Detail, C.Detail
FROM Vehicle V
LEFT OUTER JOIN Dealership A on A.Dealership = 'A' and A.id = V.id
LEFT OUTER JOIN Dealership B on B.Dealership = 'B' and B.id = V.id
LEFT OUTER JOIN Dealership C on C.Dealership = 'C' and C.id = V.id