Foreign Key constraint failure and error mesage when inserting values - sql-server

Question

Hopefully someone can help. I have created two tables Customer and Order as follows;
CREATE TABLE Customer
CustomerID int NOT NULL PRIMARY KEY
CustomerName varchar(25)
The other columns in Customer table are not relevant to my question, so I will not include them here. My CustomerID numbers are from 1 through to 15, all unique.
The second table I created is Orders as follows
CREATE TABLE Orders
OrderID smallint NOT NULL PRIMARY KEY
OrderDate date NOT NULL
CustomerID int FOREIGN KEY REFERENCES Customer (CustomerID);
My insert values is as follows
INSERT INTO Orders (OrderID, OrderDate, CustomerID)
VALUES
(1001, '2008-10-21', 1),
(1002, '2008-10-21', 8),
(1003, '2008-10-22', 15),
(1004, '2008-10-22', 5),
(1005, '2008-10-24', 3),
(1006, '2008-10-24', 2),
(1007, '2008-10-27', 11),
(1008, '2008-10-30', 12),
(1009, '2008-11-05', 4),
(1010, '2008-11-05', 1);
When I try to insert my values into the Order table, I get the following error message....
Msg 547, Level 16, State 0, Line 1.....The INSERT statement conflicted
with the FOREIGN KEY constraint "FK__OrderT__Customer__2D27B809". The
conflict occurred table "dbo.Customer", column 'CustomerID'. The
statement has been terminated.
The numbers for CustomerID in my Order table, are (1; 1; 2; 3; 4; 5; 8; 11; 12 and 15). Therefore I have checked that all my CustomerID numbers in Order table are also in the Customer table.
So my questions are
1) Has the insert values failed because my CustomerID column in Customer table in NOT NULL and I in error made CustomerID column NULL in Order.
2) If the answer to the above question is yes, then is it possible for me to (a) drop the foreign key on the CustomerID column in Order (b) change the column to NOT NULL and (c) then add the foreign key constraint again to this column and then insert the values again?
It might be easier to drop and re-create the table Order. But I am curious if option 2 would work, re dropping and adding a foreign key on the same column.
Hopefully I am on the right track with why I think the error occurred, feel
to correct me if I am wrong.
Thanks everyone
Josie

Answer 1

1) It should be NOT NULL in both. However error is because you attempted to insert a CustomerId that is not in Customer table.
2) You can simply alter the table and make it NOT NULL (error was not that).
Sample:
CREATE TABLE Customer
(
CustomerID INT NOT NULL
PRIMARY KEY ,
CustomerName VARCHAR(25)
);
CREATE TABLE Orders
(
OrderID INT NOT NULL
PRIMARY KEY ,
OrderDate DATE NOT NULL ,
CustomerID INT FOREIGN KEY REFERENCES Customer ( CustomerID )
);
INSERT [Customer] ( [CustomerID], [CustomerName] )
VALUES ( 1, 'Customer 1' ),
( 2, 'Customer 2' ),
( 3, 'Customer 3' ),
( 4, 'Customer 4' ),
( 5, 'Customer 5' ),
( 6, 'Customer 6' );
INSERT [Orders] ( [OrderID], [OrderDate], [CustomerID] )
VALUES
( 1, GETDATE(), 1 ),
( 2, GETDATE(), 2 ),
( 3, GETDATE(), 3 ),
( 4, GETDATE(), 4 ),
( 5, GETDATE(), 5 ),
( 6, GETDATE(), 6 );
INSERT [Orders] ( [OrderID], [OrderDate], [CustomerID] )
VALUES ( 7, GETDATE(), 7 );
Last one would error, because Customer with CustomerID 7 doesn't exist.
Update: I later saw your sample insert. You can find the offending ID like this:
DECLARE #ids TABLE ( id INT );
INSERT #ids ( [id] )
VALUES ( 1 ),
( 8 ),
( 15 ),
( 5 ),
( 3 ),
( 2 ),
( 11 ),
( 12 ),
( 4 ),
( 1 );
SELECT *
FROM #ids AS [i]
WHERE id NOT IN ( SELECT CustomerID
FROM [Customer] AS [c] );

Answer 2

1) Has the insert values failed because my CustomerID column in
Customer table in NOT NULL and I in error made CustomerID column NULL
in Order.
No. The error is not related to allowing NULL in the Order table. A NULL value will be allowed and not checked for referential integrity.
The foreign key violation error means you are attempting to insert a non-NULL CustomerID value into the Order table that does not exist in Customer. If you are certain the CustomerID values exist, perhaps the column mapping is wrong. Try specifying an explicit column list on the INSERT statement.

Answer 3

This error happens whern you are trying to insert a value in foreign key column, which this value does not exists in it's parent table. for example you are trying to insert value X to CustomerId in Order table, which this value does not exists in Customer table. This error occurred because we need to have a good strategy for Referential Integrity. So the only you need to do, is to check your new values(which you are going to insert them into table) to find out that is there any value compromising this rule or not.
However if you want to get an answer for your second question, you can try the below script:
create table t1
(
Id int primary key,
Name varchar(50) null
)
create table t2
(
Id int,
FK int null foreign key references t1(id)
)
go
alter table t2
alter column FK int not null

Answer 4

The identity column is set up once when you create the table.
The id assigned by an identity column start by the seed and are never reused.
So if you find that your customer ids starts from 6, it means that in the past you have added 5 customers and removed it.
If, for any reason, you want to use fixed Id don't use identity. In that case you take full responsability to set a unique value.
I suggest to never rely on fixed ids, if you must add orders from a script use the CustomerName (if unique), or any natural unique key.
You could use a script like this
DECLARE #newOrders TABLE (OrderID INT, CustomerName VARCHAR(25), OrderDate DATE);
INSERT INTO #newOrders (OrderID, CustomerName, OrderDate) VALUES
(1001, 'some-username', '2008-10-21'),
(1002, 'another-username', '2008-10-21');
INSERT INTO Orders(OrderID, CustomerId, OrderDate)
SELECT
o.OrderID,
c.CustomerID,
o.OrderDate
FROM #newOrders o
JOIN Customer c
ON c.CustomerName = o.CustomerName;
In this way you insert the correct CustomerID.
Note that in very rare cases (think twice to use it) you could insert values in identity colums using SET IDENTITY_INSERT statement.