Oracle SQL GROUP BY clause containing joins - sql

Question

I'm having issues writing this query correctly. Below is the goal, my current query, and attached are the scripts to build and populate the database. Thanks for any assistance!
For each DVD in the catalog, display it’s title, length, release_date, and how many times it has been checked out by all customers across all libraries. Include those that have not been checked out yet (display as 0). Sort results by title.
SELECT C.TITLE, D.LENGTH, C.RELEASE_DATE, COUNT(T.TRANSACTION_ID)
FROM catalog_item C
INNER JOIN dvd D ON D.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
INNER JOIN physical_item P ON P.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT OUTER JOIN transaction T ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
GROUP BY C.TITLE;
Run first: https://drive.google.com/open?id=1PYAZV4KIfZtxP4eQn35zsczySsxDM7ls
Run second: https://drive.google.com/open?id=1pAzWmJqvD3o3n6YJqVUM6TtxDafKGd3f
EDIT
I've gotten the query working but haven't figured out how to get DVDs with zero checkouts to show up. Below is my updated query.
SELECT C.TITLE, D.LENGTH, C.RELEASE_DATE, COUNT(T.TRANSACTION_ID)
FROM catalog_item C
INNER JOIN dvd D ON D.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
INNER JOIN physical_item P ON P.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT OUTER JOIN transaction T ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
GROUP BY C.TITLE, D.LENGTH, C.RELEASE_DATE;
SOLVED
Figured out the issue with GMB's help. Final query below!
SELECT C.TITLE, D.LENGTH, C.RELEASE_DATE, NVL(COUNT(T.TRANSACTION_ID), 0) AS NUMBER_OF_CHECKOUTS
FROM catalog_item C
INNER JOIN dvd D ON D.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT JOIN physical_item P ON P.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT JOIN transaction T ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
GROUP BY C.TITLE, D.LENGTH, C.RELEASE_DATE
ORDER BY C.TITLE;

Answer 1

Your second query sure looks better than the first one, as it has the correct GROUP BY clause.
It is hard to provide a 100% sure response without seeing the full tables structures, however if you are still missing records with 0 checkouts in the output, it means that one of your INNER JOINs is not matching. In other words, you have DVDs in your catalog that are either not present in the dvd table, or not present in the physical_item table. As both tables look like referential table, this could indicate a discrepency in your data. I would recommend to change all INNER JOINs to LEFT JOINs to work around this issue.
Also please note that if no checkout happened for a DVD, expression COUNT(T.TRANSACTION_ID) will yield NULL : hence you want to wrap it in a NVL function to handle this case.
New query :
SELECT C.TITLE, D.LENGTH, C.RELEASE_DATE, NVL(COUNT(T.TRANSACTION_ID), 0)
FROM catalog_item C
LEFT JOIN dvd D ON D.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT JOIN physical_item P ON P.CATALOG_ITEM_ID = C.CATALOG_ITEM_ID
LEFT JOIN transaction T ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
GROUP BY C.TITLE, D.LENGTH, C.RELEASE_DATE;