Insert current row number into SQL Server table - sql-server

Question

I have the following table in SQL Server:
CREATE TABLE [dbo].[tblTempPo](
[TempPoID] [int] IDENTITY(1,1) NOT NULL,
[guid] AS ([dbo].[GetIdentity]()),
[Qty] [int] NULL,
[MobileBrandID] [int] NULL,
[MobileID] [int] NULL
)
I need to insert the current row number to the guid column every time a new row is added. I tried to use the following function but it's not working as expected:
ALTER FUNCTION GetIdentity()
RETURNS INT AS
BEGIN
RETURN (SELECT top 1 ROW_NUMBER() OVER(ORDER BY TempPoID asc)FROM tblTempPo)
END

Answer 1

Your function GetIdentity() will probably always return 1 but it is not a sure thing because you are using select top 1... without an order by clause.
If you want the highest value returned by row_number() you need to add order by 1 desc which would be the same as doing SELECT count(*) from tblTempPo.
Fixing GetIdentity() like that will not help much in your situation because [guid] AS ([dbo].[GetIdentity]()) will give you a computed column that is evaluated every time you query the table and not when you insert a new row. You will always have the same value for all rows.
You could use a function that takes the TempPoID as a parameter as a computed column.
CREATE FUNCTION GetIdentity(#P int)
RETURNS INT AS
BEGIN
RETURN (SELECT rn
FROM (SELECT TempPoID,
ROW_NUMBER() OVER(ORDER BY TempPoID ASC) AS rn
FROM tblTempPo) AS T
WHERE TempPoID = #P)
END
Table definition:
CREATE TABLE [dbo].[tblTempPo](
[TempPoID] [int] IDENTITY(1,1) NOT NULL primary key,
[guid] as dbo.GetIdentity(TempPoID),
[Qty] [int] NULL,
[MobileBrandID] [int] NULL,
[MobileID] [int] NULL
I have never used this so I can't tell you if it is a good thing to do or not. It might be devastating for your query performance, I just don't know.

Answer 2

I agree with comments and you should change the GUID column name. If you really have to store the TempPoID column twice use computed column. Example:
CREATE TABLE #tblTempPo (
[TempPoID] [int] IDENTITY(1,1) NOT NULL,
[second_id] AS TempPoID,
[Qty] [int] NULL,
[MobileBrandID] [int] NULL,
[MobileID] [int] NULL
)
INSERT INTO #tblTempPo (Qty, MobileBrandID, MobileID) VALUES
(10, 10, 15), (20, 23, 45), (55, 23, 12), (10, 1, 1)
SELECT * FROM #tblTempPo
DROP TABLE #tblTempPo
If you need more complicated approach - use trigger.

Answer 3

It's all no good idea, but if you REALLY need it, try trigger:
create table [dbo].[tblTempPo](
[TempPoID] [int] identity(1,1) NOT NULL,
[guid] int,
[Qty] [int] NULL,
[MobileBrandID] [int] NULL,
[MobileID] [int] NULL
)
go
create trigger [dbo].[tblTempPo_Trig] on [dbo].[tblTempPo] instead of insert as
declare #cnt int
select #cnt = count(*)
from [dbo].[tblTempPo] with(nolock)
insert into [dbo].[tblTempPo]([guid], [Qty], [MobileBrandID], [MobileID])
select #cnt+row_number() over (order by [TempPoID]), [Qty], [MobileBrandID], [MobileID] from inserted
go
insert into [dbo].[tblTempPo]([Qty], [MobileBrandID], [MobileID]) values (0, 0,0), (0, 0,0), (0, 0,0), (0, 0,0)
insert into [dbo].[tblTempPo]([Qty], [MobileBrandID], [MobileID]) values (0, 0,0), (0, 0,0), (0, 0,0), (0, 0,0)
select * from [dbo].[tblTempPo]
go
drop table [dbo].[tblTempPo]
go