Why is this SQL function working differently within an insert statement? - sql

Question

I'm getting a primary key violation in the column 'voorwerpnummer'. The duplicate value is 1, but the function should never give the same number twice. When I try testing the function without anything in the 'Voorwerp' table, it returns 1, and if I test it with one row in the 'Voorwerp' table it returns 2; like it should. Why is it returning 1 twice within this insert query? I'm using SQL Server 2016.
-- Empty tables
DELETE
FROM Bestand
DELETE
FROM Voorwerp_in_rubriek
DELETE
FROM Voorwerp
-- Delete function if exists
IF EXISTS (SELECT *
FROM sys.objects
WHERE object_id = OBJECT_ID(N'[dbo].[GetVoorwerpnummer]')
AND type IN ( N'FN', N'IF', N'TF', N'FS', N'FT' ))
DROP FUNCTION [dbo].[GetVoorwerpnummer]
GO
-- Returns the next item number
CREATE FUNCTION dbo.GetVoorwerpnummer()
RETURNS INTEGER
AS BEGIN
DECLARE #Highest INTEGER
-- Find the highest number
SET #Highest = (SELECT TOP 1 voorwerpnummer FROM EenmaalAndermaal.dbo.Voorwerp ORDER BY voorwerpnummer DESC)
-- Take 0 if there is none
SET #Highest = ISNULL(#Highest, 0)
-- Add 1
SET #Highest = (#Highest + 1);
RETURN #Highest
END
GO
INSERT INTO EenmaalAndermaal.dbo.Voorwerp (looptijd, looptijdbegin, startprijs, verzendkosten, verkoopprijs, beschrijving, betalingsinstructie, betalingswijzenaam, landnaam, plaatsnaam, titel, verzendinstructies, voorwerpnummer, wel_niet_indicator, verkoper, koper)
SELECT
10 AS looptijd,
GETDATE() AS looptijdbegin,
LEFT(Prijs,8) AS startprijs,
'1,20' AS verzendkosten,
NULL AS verkoopprijs,
'Beschrijving van product' AS beschrijving,
NULL AS betalingsinstructie,
'paypal' AS betalingswijzenaam,
LEFT(land,40) AS landnaam,
LEFT(Locatie,40) AS plaatsnaam,
LEFT(Titel,40) AS titel,
NULL AS verzendinstructies,
dbo.GetVoorwerpnummer() AS voorwerpnummer,
0 AS wel_niet_indicator,
LEFT(Verkoper, 40) AS Verkoper,
NULL AS koper
FROM EBAY.dbo.Items

Answer 1

That is because SQL works set-based, not row-based.
The SELECT statement is executed first. That results in a data set, all with the same voorwerpnummer (since nothing has been added to EenmaalAndermaal.dbo.Voorwerp yet). You can test this by executing just the SELECT statement. That entire set is than INSERTed into your target table, so a subsequent call to GetVoorwerpnummer() will result in a higher voorwerpnummer (obviously if the INSERT would be successful, which it won't be if voorwerpnummer is the PK).
To correct it, I would do the following:
-- Execute function once to fill variable, so it isn't needlessly executed for every row
DECLARE #Voorwerp INT = dbo.GetVoorwerpnummer();
INSERT INTO EenmaalAndermaal.dbo.Voorwerp (looptijd, looptijdbegin, startprijs, verzendkosten, verkoopprijs, beschrijving, betalingsinstructie, betalingswijzenaam, landnaam, plaatsnaam, titel, verzendinstructies, voorwerpnummer, wel_niet_indicator, verkoper, koper)
SELECT
10 AS looptijd,
GETDATE() AS looptijdbegin,
LEFT(Prijs,8) AS startprijs,
'1,20' AS verzendkosten,
NULL AS verkoopprijs,
'Beschrijving van product' AS beschrijving,
NULL AS betalingsinstructie,
'paypal' AS betalingswijzenaam,
LEFT(land,40) AS landnaam,
LEFT(Locatie,40) AS plaatsnaam,
LEFT(Titel,40) AS titel,
NULL AS verzendinstructies,
COALESCE(#Voorwerp, 0) + ROW_NUMBER() OVER(ORDER BY id /*Choose a logical, preferably unique column here*/) AS voorwerpnummer,
0 AS wel_niet_indicator,
LEFT(Verkoper, 40) AS Verkoper,
NULL AS koper
FROM EBAY.dbo.Items
As Stilgar below commented, it is better to define the PK as an IDENTITY attribute.
Also, personally (I'm not sure if it's 'official' best practices), I prefer to keep my Primary Key columns as first columns of the table.

Answer 2

Mostly likely a matter of not committing the INSERT; until committed, the data doesn't "really" exist.
The following works for me:
CREATE TABLE dbo.PK_Test
(
PK INT
,TXT VARCHAR(0032)
);
BEGIN TRANSACTION
INSERT INTO dbo.PK_Test VALUES(05, 'First');
COMMIT TRANSACTION;
CREATE FUNCTION dbo.F_PK_Test()
RETURNS INTEGER
AS BEGIN
DECLARE #Highest INTEGER
-- Find the highest number
SET #Highest = (SELECT TOP 1 PK FROM PK_Test ORDER BY PK DESC)
-- Take 0 if there is none
SET #Highest = ISNULL(#Highest, 0)
-- Add 1
SET #Highest = (#Highest + 1);
RETURN #Highest
END;
SELECT * FROM dbo.PK_Test;
BEGIN TRANSACTION
INSERT INTO Adhoc_Area.dbo.PK_Test (PK, Txt) VALUES (Adhoc_Area.dbo.F_PK_Test(), Adhoc_Area.dbo.F_PK_Test());
INSERT INTO Adhoc_Area.dbo.PK_Test (PK, Txt) VALUES (Adhoc_Area.dbo.F_PK_Test(), Adhoc_Area.dbo.F_PK_Test());
COMMIT TRANSACTION;
SELECT * FROM dbo.PK_Test;