If Foreign Key Not Exist Then Add Foreign Key Constraint(Or Drop a Foreign Key Constraint If Exist) without using Name?

Question

I am finding difficulties to creating a a query. Let say I have a Products and Brands table. I can add a foreign key using this command,

          ALTER TABLE Pr         


        

Answer 1

First of all, you should always name your FKs and all other constraints in order to save yourself trouble like this.

But, if you don't know the name of FK you can check it using multiple system views:

IF NOT EXISTS 
(
    SELECT * FROM sys.foreign_key_columns fk 
    INNER JOIN sys.columns pc ON pc.object_id = fk.parent_object_id AND pc.column_id = fk.parent_column_id 
    INNER JOIN sys.columns rc ON rc.object_id = fk.referenced_object_id AND rc.column_id = fk.referenced_column_id
    WHERE fk.parent_object_id = object_id('Products') AND pc.name = 'BrandID'
    AND fk.referenced_object_id = object_id('Brands') AND rc.NAME = 'ID'
)
ALTER TABLE Products 
ADD CONSTRAINT Your_New_FK_NAME FOREIGN KEY (BrandID)
REFERENCES Brands(ID)

Answer 2

You can use this as well.

IF(OBJECT_ID('FK_Products_Brands', 'F') IS NULL)
ALTER TABLE [dbo].[Products] WITH CHECK ADD CONSTRAINT [FK_Products_Brands] FOREIGN KEY([BrandID]) REFERENCES [dbo].[Brands] ([Id])

Answer 3

To do this without knowing the constraint's name and without inner joins, you can do:

IF NOT EXISTS(SELECT NULL FROM INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE WHERE [TABLE_NAME] = 'Products' AND [COLUMN_NAME] = 'BrandID')
BEGIN
    ALTER TABLE Products
    ADD FOREIGN KEY (BrandID)
    REFERENCES Brands(ID)
END

If you wanted to, you could get the name of the contraint from this table, then do a drop/add.

Answer 4

Try this:

IF NOT EXISTS (SELECT * FROM sys.objects o WHERE o.object_id = object_id(N'[dbo].[FK_Products_Brands]') AND OBJECTPROPERTY(o.object_id, N'IsForeignKey') = 1)
BEGIN
    ALTER TABLE [dbo].[Products] WITH CHECK ADD CONSTRAINT [FK_Products_Brands] FOREIGN KEY([BrandID]) REFERENCES [dbo].[Brands] ([Id])
END