Oracle SQL How to Count Column Value Occurences and Group BY during joins

Question

I'm working on another SQL query, trying to group a collection of records while doing a count and joining tables. See below for goal, current query, and attached scripts for building and populating tables.

Show all customers who have checked more books than DVDs. Display customer name, total book checkouts and total DVD checkouts. Sort results by customer first name and last name.

SELECT C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME, COUNT(T.TRANSACTION_ID)
FROM customer C
INNER JOIN library_card LC ON C.CUSTOMER_ID = LC.CUSTOMER_ID
INNER JOIN transaction T ON LC.LIBRARY_CARD_ID = T.LIBRARY_CARD_ID
INNER JOIN physical_item P ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
INNER JOIN catalog_item CT ON P.CATALOG_ITEM_ID = CT.CATALOG_ITEM_ID 
GROUP BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME
ORDER BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME;

Run first: https://drive.google.com/open?id=1PYAZV4KIfZtxP4eQn35zsczySsxDM7ls

Run second: https://drive.google.com/open?id=1pAzWmJqvD3o3n6YJqVUM6TtxDafKGd3f

EDIT
With some help from Mr. Barbaros I've come up with the below query, which is closer. However, this query isn't returning any results for DVDs, which leads me to believe it's a join issue.

SELECT C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME, COUNT(CT1.TYPE) AS BOOK_COUNT, COUNT(CT2.TYPE) AS DVD_COUNT
FROM customer C
INNER JOIN library_card LC ON C.CUSTOMER_ID = LC.CUSTOMER_ID
INNER JOIN transaction T ON LC.LIBRARY_CARD_ID = T.LIBRARY_CARD_ID
INNER JOIN physical_item P ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
INNER JOIN catalog_item CT1 ON P.CATALOG_ITEM_ID = CT1.CATALOG_ITEM_ID AND CT1.TYPE = 'BOOK'
LEFT OUTER JOIN catalog_item CT2 ON P.CATALOG_ITEM_ID = CT2.CATALOG_ITEM_ID AND CT2.TYPE = 'DVD'
GROUP BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME, CT1.TYPE, CT2.TYPE
ORDER BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME;

Answer 1

Use "conditional aggregates" (use a case expression inside the aggregate function)

SELECT
    C.CUSTOMER_FIRSTNAME
  , C.CUSTOMER_LASTNAME
  , COUNT( CASE WHEN CT.TYPE = 'BOOK' THEN T.TRANSACTION_ID END ) books
  , COUNT( CASE WHEN CT.TYPE = 'DVD' THEN T.TRANSACTION_ID END ) dvds
FROM customer C
INNER JOIN library_card LC ON C.CUSTOMER_ID = LC.CUSTOMER_ID
INNER JOIN transaction T ON LC.LIBRARY_CARD_ID = T.LIBRARY_CARD_ID
INNER JOIN physical_item P ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
INNER JOIN catalog_item CT ON P.CATALOG_ITEM_ID = CT.CATALOG_ITEM_ID
GROUP BY
    C.CUSTOMER_FIRSTNAME
  , C.CUSTOMER_LASTNAME
HAVING
     COUNT( CASE WHEN CT.TYPE = 'BOOK' THEN T.TRANSACTION_ID END ) 
   > COUNT( CASE WHEN CT.TYPE = 'DVD' THEN T.TRANSACTION_ID END )
ORDER BY
    C.CUSTOMER_FIRSTNAME
  , C.CUSTOMER_LASTNAME
;

Answer 2

You can use catalog_item table twice( think of as seperate tables for books and dvds ), and compare by HAVING clause as :

SELECT C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME, 
       COUNT(CT1.CATALOG_ITEM_ID) as "Book Checkout", 
       COUNT(CT2.CATALOG_ITEM_ID) as "DVD Checkout"
  FROM customer C
 INNER JOIN library_card LC ON C.CUSTOMER_ID = LC.CUSTOMER_ID
 INNER JOIN transaction T ON LC.LIBRARY_CARD_ID = T.LIBRARY_CARD_ID
 INNER JOIN physical_item P ON T.PHYSICAL_ITEM_ID = P.PHYSICAL_ITEM_ID
  LEFT JOIN catalog_item CT1 ON P.CATALOG_ITEM_ID = CT1.CATALOG_ITEM_ID AND CT1.TYPE = 'BOOK'
  LEFT JOIN catalog_item CT2 ON P.CATALOG_ITEM_ID = CT2.CATALOG_ITEM_ID AND CT1.TYPE = 'DVD'
 GROUP BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME
HAVING COUNT(CT1.CATALOG_ITEM_ID) > COUNT(CT2.CATALOG_ITEM_ID)
 ORDER BY C.CUSTOMER_FIRSTNAME, C.CUSTOMER_LASTNAME;

CUSTOMER_FIRSTNAME  CUSTOMER_LASTNAME   Book Checkout  DVD Checkout
------------------  -----------------   -------------  -------------
Deena               Pilgrim                   3              1
Emile               Cross                     5              2

Answer 3

请尝试在group by子句中删除CT1.TYPE，CT2.TYPE 。