Update count column from data in another table - sql-server

Question

In my DB I have two tables Items(Id, ..., ToatlViews int) and ItemViews (id, ItemId, Timestamp)
In ItemViews table I store all views of an item as they come to the site. From time to time I want to call a stored procedure to update Items.ToatlViews field. I tried to do this SP using a cursor ... but the update statement is wrong. Can you help me to correct it? Can I do this without cursor?
CREATE PROCEDURE UpdateItemsViews
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
DECLARE #currentItemId int
DECLARE #currentItemCursor CURSOR
SET #currentItemCursor = CURSOR FOR SELECT Id FROM dbo.Items
OPEN #currentItemCursor
FETCH NEXT FROM #currentItemCursor INTO #currentItemId
WHILE ##FETCH_STATUS = 0
BEGIN
Update dbo.Items set TotalViews = count(*)
from dbo.ItemViews where ItemId=#currentItemId
FETCH NEXT FROM #currentItemCursor INTO #currentItemId
END
END
GO

Answer 1

You can use a direct UPDATE statement
update Items set TotalViews =
(select COUNT(id) from ItemViews where ItemViews.ItemId = Items.Id)
You might want to test performance for the various ways to do this, if that's important.

Answer 2

You could use update ... from instead of a cursor:
update i
set TotalViews = iv.cnt
from dbo.Item i
join (
select ItemId
, count(*) as cnt
from dbo.ItemViews
group by
ItemId
) iv
on i.Id = iv.ItemId

Answer 3

;WITH x AS
(
SELECT ItemID, c = COUNT(*)
FROM dbo.ItemViews
GROUP BY ItemID
)
UPDATE i
SET TotalViews = x.c
FROM dbo.Items AS i
INNER JOIN x
ON x.ItemID = i.ItemID;
But why do you want to store this value, when you can always get the count at runtime? You're going to have to run this update statement every time you touch the ItemViews table in any way, otherwise the count stored with Items is going to be incorrect.
What you may consider doing instead is setting up an indexed view:
CREATE VIEW dbo.ItemViewCount
WITH SCHEMABINDING
AS
SELECT ItemID, ItemCount = COUNT_BIG(*)
FROM dbo.ItemViews
GROUP BY ItemID;
GO
CREATE UNIQUE CLUSTERED INDEX x ON dbo.ItemViewCount(ItemID);
Now you can join to the view in your queries and know that the count is always up to date (without paying the penalty of scanning for the count of each item). The downside to the indexed view is that you pay that cost incrementally when there are inserts/updates/deletes to the ItemViews table.

Answer 4

I found this question / answer a year after it was written and answered. the answer was okay, but I was after something a bit more automatic. I ended up writing a trigger to automatically recalculate the column when a relevant row in the other table was inserted, deleted or updated.
I think it's a better solution than running something manually to do the recalculation as there isn't any possibility of someone forgetting to run the code:
CREATE TRIGGER [dbo].[TriggerItemTotalViews]
ON [dbo].[ItemViews]
AFTER INSERT, DELETE, UPDATE
AS
BEGIN
SET NOCOUNT ON;
UPDATE [Items]
SET [TotalViews] =
(
SELECT COUNT(id)
FROM [ItemViews]
WHERE [ItemViews].[ItemId] = [Items].[ItemId]
)
WHERE [Items].[ItemId] IN
(
SELECT [ItemId] FROM [INSERTED]
UNION
SELECT [ItemId] FROM [DELETED]
)
END

Answer 5

Same but different:
declare #productId int = 24;
declare #classificationTypeId int = 86;
update s
set CounterByProductAndClassificationType = row_num
from Samples s
join
(
select row_number() over (order by (select Id)) row_num, Id
from Samples
where
ProductId = #productId and
ClassificationTypeId = #classificationTypeId
) s_row on s.Id = s_row.Id

Answer 6

For who need to include zero count too
UPDATE Items as i,
(SELECT
i.Id as Id, COUNT(iv.ItemId) AS c
FROM
Items AS i
LEFT JOIN ItemViews AS iv ON i.Id = iv.ItemId
GROUP BY i.Id) AS ic
SET
i.TotalViews = ic.c
WHERE
i.Id = ic.Id