Update count column from data in another table

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

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

Answer 2

;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 3

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 4

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.