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.)