SQL Query to Update Object Count Based on Event Name - sql

Question

Imagine I am an owner of many bookstores. I keep a database of all events that occur in all of my many bookstores. Two events of note are "Book Added" and "Book Removed", for when a book is added to the inventory of a story, and when it is sold from a store. An example schema would be bookstore_id, event_name, `time.
Now say I have a second table, which maintains the current state of each bookstore, so the schema would be bookstore_id, num_books.
I want to be able to use the first table to get the count of all the "Book Added" events per bookstore, subtract the count of all the "Book Removed" events per bookstore, and then update the number of books in each bookstore in the second table.
The only way I can think to do it requires using a cursor, but I'm assuming there's a more "SQL-esque" way to do it that is more set-based and doesn't require a cursor.

Answer 1

You can count the events by using a GROUP BY clause.
If we would create 2 subtables where we count the added respectively the removed books, we can simply subtract the results and update these in the parent table. This will look like:
UPDATE b
SET b.numbooks = AddedBooks.BooksAdded - RemovedBooks.BooksRemoved
FROM dbo.Books b
INNER JOIN (SELECT be.book_id, count(*) AS BooksAdded
FROM dbo.BookEvents be
WHERE be.event = 'BookAdded'
GROUP BY be.book_id, be.event) AS AddedBooks
ON b.bookid = AddedBooks.book_id
INNER JOIN (SELECT be.book_id, count(*) AS BooksRemoved
FROM dbo.BookEvents be
WHERE be.event = 'BookRemoved'
GROUP BY be.book_id, be.event) AS RemovedBooks
ON b.bookid = RemovedBooks.book_id

Answer 2

select bookstore_id
, sum(case when event_name = "Book Removed" then -1 else 1 end) as "num books"
from bookstores
group by bookstore_id
if more than 2 events
select bookstore_id
, sum(case when event_name = "Book Removed" then -1
when event_name = "Book Added" then 1
end) as "num books"
from bookstores
group by bookstore_id
And I would just make it a view unless you come up with performance issues

Answer 3

We can use CTEs to get details individually and process them.
With CTE_Add AS
(
Select Bkstr_ID, Count(event_Name) As Added From temp Where event = 'Added' Group by Bkstr_ID
), CTE_Rem As
(
Select Bkstr_ID, Count(event_Name) As Removed From temp Where event = 'Removed' Group by Bkstr_ID
)
Select A.Bkstr_ID, Added - Removed
From CTE_Add A
Left Join CTE_Rem R On A.Bkstr_ID= R.Bkstr_ID
This will give you ID and count.
Instead of select, you can use Insert statement

Answer 4

I'd use SUM(CASE WHEN ...). Below is an example.
If object_id('tempdb..#BoookStores') Is Not Null Drop Table #BoookStores
Create Table #BoookStores (bookstore_id int, num_books int)
/* We have 3 stores */
Insert #BoookStores (bookstore_id, num_books)
Values (1, 0), (2, 0), (3, 0)
If object_id('tempdb..#Events') Is Not Null Drop Table #Events
Create Table #Events (bookstore_id int, event_name varchar(10), time dateTime Default(GetDate()) )
Insert #Events (bookstore_id, event_name)
Values
(1, 'Added'), (1, 'Added'), (1, 'Added'), (1, 'Added'), -- Added 4 books to 1. store
(2, 'Added'), (2, 'Added'), (2, 'Added'), -- Added 3 books to 2. store
(3, 'Added'), (3, 'Added'), -- Added 2 books to 3. store
/* removed 2 books from each stores */
(1, 'Removed'), (1, 'Removed'),
(2, 'Removed'), (2, 'Removed'),
(3, 'Removed'), (3, 'Removed')
/* Calculate adds and removes. Update the results */
;With Tmp As (
Select E.bookstore_id,
Sum(Case When E.event_name = 'Added' Then 1 Else 0 End) As AddCount,
Sum(Case When E.event_name = 'Removed' Then 1 Else 0 End) As RemoveCount
From #Events E
Group By E.bookstore_id
)
Update BS Set num_books = T.AddCount-T.RemoveCount
From #BoookStores BS
Inner Join Tmp T On T.bookstore_id = BS.bookstore_id
/* check results*/
Select * From #BoookStores BS

Answer 5

Something like this will get you in the ball park. Similar logic could be used for INSERT.
UPDATE tableA
SET tableA.num_books = tableB.num_books
FROM secondTable AS TableA
INNER JOIN (
SELECT bookstore_id,
SUM(CASE
WHEN event_name = 'Books Added'
THEN 1
END) - SUM(CASE
WHEN event_name = 'Books Removed'
THEN 1
END
) AS num_books
FROM firstTable
GROUP BY bookstore_id
) TableB ON TableA.bookstore_id = tableB.bookstore_id

Answer 6

You can try a query like below:
update t1
set num_books=inventory
FROM bs t1 LEFT JOIN
(select bookstore_id,SUM(case when event_name like 'A' then 1 when event_name like 'R' then -1 else NULL end) as inventory
from bse
group by bookstore_id) t2
on t1.bookstore_id=t2.bookstore_id
Live SQL demo

Answer 7

UPDATE bsc
SET bsc.num_books = bse.num_books
FROM bookstorecounts bsc
JOIN (SELECT bookstore_id,
SUM(CASE event_name
WHEN 'Book Removed' THEN -1
WHEN 'Book Added' THEN 1
END) AS num_books
FROM bookstoreevents
GROUP BY bookstore_id
) bse ON bsc.bookstore_id = bse.bookstore_id