UPDATE doesn't allow to INSERT null in FK field - sql-server

Question

I have a SQL Server database and in a table there's a lookup column which is a "nullable FK" linked to a master table.
There's a data import process in which we fetch data from table01_staging into table01. This is the UPDATE statement which is failing (if CarrierID is null) -
DECLARE #code as nvarchar(10); SET #code = 'xyz';
UPDATE table01
SET CarrierID = t2.CarrierID
FROM table01 AS t1
INNER JOIN table01_staging AS t2 ON t1.Code = #code;
WHERE t1.Code = #code;
It goes fine if the CarrierID is not null. In fact I can successfully execute:
UPDATE table01
SET CarrierID = null
WHERE t1.Code = 'xyz';
So setting null is not the problem but it doesn't work when its updated from the staging table which has a null value. How can I make it right?
Error : The UPDATE statement conflicted with the FOREIGN KEY constraint FK_table01_MasterCarrier. The conflict occurred in database table01, table dbo.MasterCarrier, column ID.
> EDIT 02: DONE!. Updated the first SQL to show my variable usage - which I believe was the culprit. Correcting the JOIN operation as follows with a COLLATion conversion error fix makes it work -
DECLARE #code as nvarchar(10); SET #code = 'xyz';
UPDATE table01
SET CarrierID = t2.CarrierID
FROM table01 AS t1
INNER JOIN table01_staging AS t2 ON t1.Code = t2.code COLLATE SQL_Latin1_General_CP1_CI_AS
WHERE t1.Code = #code;
Thank you all, esp. Zhang. I deserve a -1 for not being able to understand the JOIN clause properly :-)

Answer 1

Does table01_staging have the same Foreign Key Constraint on CarrierID?
If not, check whether there are some CarrierID not existing in MasterCarrier table.
select * from table01_staging t
where not exists (select 1 from MasterCarrier m where m.ID = t.CarrierID)
!!UPDATE!!
I created the sample table and data. It worked without any issue.
CREATE TABLE [dbo].MasterCarrier(
[id] [int] IDENTITY NOT NULL,
[NAME] [varchar](10) NULL,
CONSTRAINT [PK_mastertable] PRIMARY KEY CLUSTERED
(
[id] ASC
)
)
GO
CREATE TABLE [dbo].[table01](
[id] [int] NOT NULL,
[CarrierID] [int] NULL,
[Code] [varchar](10) NULL
)
GO
ALTER TABLE [dbo].table01 WITH CHECK ADD CONSTRAINT [FK_table01_MasterCarrier] FOREIGN KEY([CarrierID])
REFERENCES [dbo].[MasterCarrier] ([id])
GO
ALTER TABLE [dbo].table01 CHECK CONSTRAINT [FK_table01_MasterCarrier]
GO
CREATE TABLE [dbo].[table01_staging](
[id] [int] NOT NULL,
[CarrierID] [int] NULL,
[Code] [varchar](10) NULL
)
GO
--Insert sample data
INSERT INTO MasterCarrier (NAME) VALUES ('carrier');
INSERT INTO table01 (id, CarrierID, Code) VALUES (1, 1, 'abc'), (2, NULL, 'abc'),(3, 1, 'ddd');
INSERT INTO table01_staging (id, CarrierID, Code) VALUES (1, 1, 'abc'), (2, NULL, 'abc'),(3, 1, 'ddd');
UPDATE table01
SET CarrierID=t2.CarrierID
FROM table01 AS t1
INNER JOIN table01_staging AS t2 ON t1.ID = t2.ID
WHERE t1.Code='abc'
It gets (2 rows affected) message.