SQL Server 2008: Administering Database Objects - Working with Tables (part 3) - Foreign Key Constraints

4. Foreign Key Constraints

You can create a foreign key to enforce a relationship between the data in two tables. Say that you have an Orders table and an OrderDetails table. When a customer places an order, you insert one record into Orders. Then, for each line item, you place a record into OrderDetails. Naturally you would want each line item in OrderDetails to contain the order number, so that you could refer to the order record for any given line item. That order number in OrderDetails is an example of a foreign key. By defining such a relationship explicitly, you enable SQL Server to ensure that each order number in the child table OrderDetails represents a valid order in the parent table Orders.

To create a foreign key you can use the ALTER TABLE statement. You can use the NO CHECK option to force the creation of the foreign key, even if the existing data does not meet the foreign key requirements. All data entered after the foreign key is created will then be forced comply with the constraint. Listing 9-15 uses the ALTER TABLE statement to create a foreign key constraint on the OrderID column in the OrderDetails table that references the OrderID column in the Orders table.

You can specify the CASCADE option to apply any updates or deletes made to the Orders table to the OrderDetails table. For example, if an order is deleted from the Orders table, all of the records in the OrderDetails table that reference that OrderID will be deleted as well.

Example 11. Creating a Foreign Key Constraint Between Two Tables

USE AdventureWorks2008
GO

--Drop the tables if they currently exist
IF OBJECT_ID('dbo.OrderDetails', 'U') IS NOT NULL
  DROP TABLE dbo.OrderDetails;

IF OBJECT_ID('dbo.Orders', 'U') IS NOT NULL
  DROP TABLE dbo.Orders;

--Create the tables
CREATE TABLE Orders
(OrderID int NOT NULL CONSTRAINT PK_ORDERS PRIMARY KEY CLUSTERED,
OrderNumber int NULL CONSTRAINT UQ_ORDER_NUMBER UNIQUE NONCLUSTERED)

CREATE TABLE OrderDetails
(OrderDetailID int IDENTITY (1,1) NOT NULL
CONSTRAINT PK_ORDER_DETAILS PRIMARY KEY CLUSTERED,
OrderID int NOT NULL,
ProductID int NOT NULL)

--Add the foreign key constraint
ALTER TABLE OrderDetails WITH NOCHECK
ADD CONSTRAINT FK_OrderDetails_Orders FOREIGN KEY(OrderID)
REFERENCES Orders (OrderID)
ON UPDATE CASCADE
ON DELETE CASCADE

					  

The script from Listing 11 creates the tables shown in Figure 2 with a one-to-many relationship between the Orders and the OrderDetails tables.

Figure 2. Foreign key relationship between the Orders and OrderDetails tables

You can disable a foreign key when you want to perform certain operations, such as inserts and deletes that the foreign key would cause to fail. Execute the ALTER TABLE statement with the NOCHECK CONSTRAINT keywords followed by the constraint name. Optionally, you can use the ALL keyword to disable all constraints on a table at once. Re-enable the constraints by changing the NOCHECK keyword to CHECK and reissuing the statement. Listing 12 shows the syntax to disable and enable constraints.

Example 12. Syntax Used to Disable/Enable Constraints

--Disable the FK_OrderDetails_Orders constraint on the OrderDetails table
ALTER TABLE OrderDetails NOCHECK CONSTRAINT FK_OrderDetails_Orders

--Disable the all constraints on the OrderDetails table
ALTER TABLE OrderDetails NOCHECK CONSTRAINT ALL

--Enable the FK_OrderDetails_Orders constraint on the OrderDetails table
ALTER TABLE OrderDetails CHECK CONSTRAINT FK_OrderDetails_Orders

--Enable the all constraints on the OrderDetails table
ALTER TABLE OrderDetails CHECK CONSTRAINT ALL

To remove a foreign key constraint from a table, issue the ALTER TABLE statement with the DROP CONSTRAINT keywords followed by the name of the foreign key constraint, as shown in Listing 13.

Example 13. Code to Remove a Foreign Key Constraint

ALTER TABLE dbo.OrderDetails DROP CONSTRAINT FK_OrderDetails_Orders