SQL Trigger throws error but still inserts - sql

Question

I am working on a trigger that is supposed to block an insert when #verkoperstatus is 0; this does function, but for some reason it also stops the insert when #verkoperstatus is 1. What could be the root cause behind this?
CREATE TRIGGER [dbo].[verkoper_check] ON [dbo].[Verkoper]
FOR INSERT,UPDATE
AS
BEGIN
DECLARE #verkoperstatus bit
DECLARE #gebruikersnaam varchar(25)
SELECT #gebruikersnaam = gebruikersnaam FROM inserted
SELECT #verkoperstatus = verkoper FROM Gebruiker WHERE gebruikersnaam = #gebruikersnaam
IF #verkoperstatus = 0
BEGIN
RAISERROR('Geen verkoper!',18,1);
ROLLBACK;
END
ELSE
BEGIN
COMMIT;
END
END
It should insert when #verkoperstatus is 1, and raise an error when #verkopstatus is 0.
The table Gebruiker is references, which includes a 'gebruikersnaam' and a 'verkoper' column. The value of the 'gebruikersnaam' column is the identifying column which (in this specific case is 'Lars'). Verkoper is a bit column, which indicated if one is a seller or not, so this has the value of a 0 or a 1.
The goal I am trying to achieve is to have an insert on the Verkoper tabel if a 'gebruikersnaam' has the 'verkoper' value of one.
This means if there is a row in Gebruiker with the 'gebruikersnaam' of Lars and the verkoper has a value of 1. This will be an allowed insert into the Verkoper tabel.
As the Verkoper has the following columns: 'gebruikersnaam', 'banknaam', 'rekeningnummer', 'controleoptienaam' and 'creditcardnummer'. When 'gebruikersnaam' corresponds with a 'gebruikersnaam' from the Gebruikers table AND has a value of 1 in the 'verkoper' column this record will be allowed to be inserted into the Verkoper table.
As of now there is a row in the Gebruikers column which includes the gebruikersnaam 'Lars' and a verkoper value of '1'. Meaning any SQL Insert with the gebruikersnaam of 'Lars' should be allowed into the Verkoper table.
This however does not function the way I believe it should.
These are the tables mentioned above:
CREATE TABLE Verkoper (
gebruikersnaam varchar(25) NOT NULL,
banknaam varchar(255) NULL,
rekeningnummer varchar(32) NULL,
controleoptienaam char(10) NOT NULL,
creditcardnummer integer NULL,
CONSTRAINT pk_Verkoper PRIMARY KEY (gebruikersnaam),
CONSTRAINT fk_Verkoper_Gebruikersnaam FOREIGN KEY (gebruikersnaam) REFERENCES Gebruiker(gebruikersnaam),
CONSTRAINT ck_rekening CHECK (rekeningnummer is NOT NULL OR creditcardnummer is NOT NULL),
CONSTRAINT ck_controleoptie CHECK (controleoptienaam IN('Post', 'Creditcard'))
)
CREATE TABLE Gebruiker(
gebruikersnaam varchar(25) NOT NULL,
voornaam varchar(25) NOT NULL,
achternaam varchar(25) NOT NULL,
adresregel_1 varchar(255) NULL,
adresregel_2 varchar(255) NULL,
postcode char(7) NULL,
plaatsnaam varchar(255) NULL,
land varchar(255) NULL,
geboortedag char(10) NOT NULL,
mailbox varchar(255) NOT NULL,
wachtwoord varchar(255) NOT NULL,
verkoper bit NOT NULL,
CONSTRAINT pk_gebruiker PRIMARY KEY (gebruikersnaam),
)
For the inserts I am using the following data:
INSERT INTO Gebruiker VALUES ('Lars', 'Lars', 'Last_name', null, null, null, null, null, '04/04/2019', 'lars#mymailbox.cloud', 'MyPassword', 1)
INSERT INTO Verkoper VALUES ('Lars', 'ING', 'NL32ABN32492809', 'Post', null)

Answer 1

This is untested, however, I suspect this is the logic you really need:
CREATE TRIGGER [dbo].[verkoper_check] ON [dbo].[Verkoper]
FOR INSERT,UPDATE
AS BEGIN
IF EXISTS(SELECT 1
FROM inserted i
JOIN Gebruiker G ON i.gebruikersnaam = G.gebruikersnaam
WHERE G.verkoper = 0) BEGIN
RAISERROR('Geen verkoper!',18,1);
ROLLBACK;
END;
END;