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USING A FOREIGN KEY WITH A SUBQUERY
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Let s implement the validation of the phone number as a combination of a check constraint and a foreign key constraint. You use the first version of the UDF (the one without the subquery) with a foreign key. How can you implement the foreign key You want it to check the area code only against the Test.AreaCodes table, not the entire phone number. You do this by implementing a computed column that returns only the area code portion of the phone number. You need to do a couple of things to make it possible to create the foreign key shown in the example. The result of the expression in the AreaCode column must be of the same data type as the column that the foreign key references, CHAR(3). You ensure this by calling the CAST function in the AreaCode expression.
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110 CHAPTER 3 Tables, Data Types, and Declarative Data Integrity
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The column must also be marked as PERSISTED, which means that SQL Server physically stores the result of the computed column s expression in the data row instead of calculating it each time it is referenced in a query. It is recalculated every time the column is updated. One of the reasons for this requirement is that it affects performance; you don t want SQL Server to execute the SUBSTRING function each time the foreign key needs to be validated. The following script creates the new version of the Test.Contacts table, including the added foreign key constraint:
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CREATE TABLE Test.Contacts ( ContactID INT IDENTITY PRIMARY KEY ,Name NVARCHAR(50) NOT NULL ,PhoneNumber VARCHAR(20) NULL ,CONSTRAINT CKContactsPhoneNumber CHECK(dbo.fnIsPhoneNumber(PhoneNumber) = 1) ,AreaCode AS CAST(SUBSTRING(PhoneNumber, 2, 3) AS CHAR(3)) PERSISTED ,CONSTRAINT FKContactsAreaCodes FOREIGN KEY (AreaCode) REFERENCES Test.AreaCodes );
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As you can see, the AreaCode column in the Contacts table is just a subset of the PhoneNumber column. What happens if you insert a NULL value into the PhoneNumber column The SUBSTRING function returns NULL, and NULL is accepted by the foreign key and interpreted as a value that does not reference the AreaCodes table.
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CHECK AND FOREIGN KEY CONSTRAINTS VS. QUERY PERFORMANCE
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Can check and foreign key constraints improve query performance Don t they just protect us against invalid data, and in doing so, somewhat degrade performance The answers to these questions are Yes, they can, and No, they don t. Because foreign keys and check constraints are declared rules, the optimizer can use them to create more efficient query plans. This usually involves skipping some part of the query plan because for example, the optmizer can see that because of a foreign key constraint, it is unnecessary to execute that particular part of the plan. The following code sample is a simple example of this behavior with a foreign key constraint. Consider the following two tables and the foreign key FKOrdersCustomers:
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CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY );
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CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NOT NULL CONSTRAINT FKOrdersCustomers REFERENCES Test.Customers (CustomerID) );
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Lesson 2: Declarative Data Integrity
Now, let s look at what SQL Server actually does when you query these tables with the foreign key in place. To do this, in SSMS, from the Query menu, choose Include Actual Execution Plan, or, alternatively, press Ctrl-M. The following query returns all orders that have a valid customer reference:
SELECT o.* FROM Test.Orders AS o WHERE EXISTS (SELECT * FROM Test.Customers AS c WHERE c.CustomerID = o.CustomerID);
The execution plan that SQL Server uses to execute this query is shown in Figure 3-1. In the execution plan, you can see that the Test.Customers table is not accessed; the only table being accessed is Test.Orders. This is because the optimizer knows that it is not necessary to execute the EXISTS operator in this query because the foreign key constraint requires all orders to refer to an existing customer, which is what the WHERE clause checks.
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