如何将外键添加到SQL Server 2012中的现有列

Question

I am trying to add foreign key to my existing column using below query

ALTER TABLE Sub_Category_Master 
ADD FOREIGN KEY (Category_ID) REFERENCES Category_Master(Category_ID)

but I'm getting an error

The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK__Sub_Categ__Categ__5812160E". The conflict occurred in database "shaadikarbefikar_new", table "shaadikarbefikar.Category_Master", column 'Category_ID'.

Answer 1

Well, the error clearly tells you that Category_ID in your Sub_Category_Master table contains some values that are not present in Category_Master (column Category_ID ). But that's exactly the point of having a foreign key constraint - making sure your child table ( Sub_Category_Master ) only uses defined values from its parent table.

Therefore, you must fix those "voodoo" values first, before you're able to establish this foreign key relationship. I would also strongly recommend to explicitly name that constraint yourself, to avoid those system-generated, but not really very useful constraint names like FK__Sub_Categ__Categ__5812160E :

ALTER TABLE Sub_Category_Master 
ADD CONSTRAINT FK_SubCategoryMaster_CategoryMaster
    FOREIGN KEY (Category_ID) REFERENCES Category_Master(Category_ID)

Answer 2

ALTER TABLE Sub_Category_Master
ADD CONSTRAINT FKSub_Category_Master_Category_ID FOREIGN KEY (Category_ID)
    REFERENCES  Category_Master(Category_ID);

Answer 3

CREATE TABLE Orders 
(
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),

    CONSTRAINT FK_PersonOrder 
        FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);