Sql Server select and update X rows - sql

Question

I'm using Sql Server 2012.
I need to select rows from a table for processing. The number of rows needs to be variable. I need to update the rows I'm selecting to a "being processed" status - I have a guid to populate for this purpose.
I've encountered several examples of using row_number() and a couple of examples of ways of using CTE's, but I'm not sure on how to combine them (or if that's even the correct strategy). I would appreciate any insight.
Here is what I have so far:
DECLARE #SessionGuid uniqueidentifier, #rowcount bigint
SELECT #rowcount = 1000
SELECT #sessionguid = newid()
DECLARE #myProductChanges table (
ProductChangeId bigint
, ProductTypeId smallint
, SourceSystemId tinyint
, ChangeTypeId tinyint );
WITH NextPage AS
(
SELECT
ProductChangeId, ServiceSessionGuid,
ROW_NUMBER() OVER (ORDER BY ProductChangeId) AS 'RowNum'
FROM dbo.ProductChange
WHERE 'RowNum' < #rowcount
)
UPDATE dbo.ProductChange
SET ServiceSessionGuid = #sessionguid, ProcessingStateId = 2, UpdatedDate = getdate()
OUTPUT
INSERTED.ProductChangeId,
INSERTED.ProductTypeId,
INSERTED.SourceSystemId,
INSERTED.ChangeTypeId
INTO #myProductChanges
FROM dbo.ProductChange as pc join NextPage on pc.ProductChangeId = NextPage.ProductChangeId
From here I will select from my temp table and return the data:
SELECT mpc.ProductChangeId
, pt.ProductName as ProductType
, ss.Name as SourceSystem
, ct.ChangeDescription as ChangeType
FROM #myProductChanges as mpc
join dbo.R_ProductType pt on mpc.ProductTypeId = pt.ProductTypeId
join dbo.R_SourceSystem ss on mpc.SourceSystemId = ss.SourceSystemId
join dbo.R_ChangeType ct on mpc.ChangeTypeId = ct.ChangeTypeId
ORDER BY ProductType asc
So far this doesn't work for me. I get an error when I try to run it:
Msg 8114, Level 16, State 5, Line 20
Error converting data type varchar to bigint.
I'm not clear on what I'm doing wrong - so - any help is appreciated.
Thanks!
BTW, here are some of the questions I've used as reference to try and solve this:
https://stackoverflow.com/questions/9777178
https://stackoverflow.com/questions/3319842
https://stackoverflow.com/questions/6402103

Answer 1

This subquery makes no sense:
SELECT
ProductChangeId, ServiceSessionGuid,
ROW_NUMBER() OVER (ORDER BY ProductChangeId) AS 'RowNum'
FROM dbo.ProductChange
WHERE 'RowNum' < #rowcount
You can't reference the alias RowNum at the same scope (and you are trying to compare a string, not an alias, anyway), because when the WHERE clause is parsed, the SELECT list hasn't been materialized yet. What you need is either another nest:
SELECT ProductChangeId, ServiceSessionGuid, RowNum
FROM (SELECT ProductChangeId, ServiceSessionGuid,
ROW_NUMBER() OVER (ORDER BY ProductChangeId) AS RowNum
FROM dbo.ProductChange
) AS x WHERE RowNum < #rowcount
Or:
SELECT TOP (#rowcount-1) ProductChangeId, ServiceSessionGuid,
ROW_NUMBER() OVER (ORDER BY ProductChangeId) AS RowNum
FROM dbo.ProductChange
ORDER BY ProductChangeId
Also please stop using 'alias' - when you need to delimit aliases (you don't in this case), use [square brackets].

Answer 2

I'm guessing, but I think you want <= rather than < if you want to affect #rowcount rows, not one less.
Another tip is that CTEs can be updated directly*, as shown here:
WITH NextPage AS
(
SELECT TOP(#rowcount) *
FROM dbo.ProductChange
)
UPDATE NextPage
SET ServiceSessionGuid = #sessionguid, ProcessingStateId = 2, UpdatedDate = getdate()
OUTPUT
INSERTED.ProductChangeId,
INSERTED.ProductTypeId,
INSERTED.SourceSystemId,
INSERTED.ChangeTypeId
INTO #myProductChanges
* The updates affect the base table in the CTE, i.e. dbo.ProductChange