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Tables, Data Types, and Declarative Data Integrity
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In the previous example, consider what happens if a customer is deleted all the customer s orders are also deleted. This might be fine, but consider the following code:
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CREATE TABLE Test.Countries ( CountryID INT PRIMARY KEY );
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CREATE TABLE Test.Cities ( CityID INT PRIMARY KEY ,CountryID INT NOT NULL REFERENCES Test.Countries ON DELETE CASCADE );
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CREATE TABLE Test.Customers ( CustomerID INT PRIMARY KEY ,CityID INT NOT NULL REFERENCES Test.Cities ON DELETE CASCADE );
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CREATE TABLE Test.Orders ( OrderID INT PRIMARY KEY ,CustomerID INT NOT NULL REFERENCES Test.Customers ON DELETE CASCADE );
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In this example, if you delete a country, all cities in that country, all customers in those cities, and all orders belonging to those customers are also deleted. Be cautious you might be deleting more than you think. Consider someone executing the query DELETE Test.Countries WHERE CountryID = 1; from SSMS. The person might think he is deleting only one row in the Countries table, when he or she might actually be deleting millions of rows. The time it takes to execute this DELETE statement depends on how many rows are being deleted. When it finishes, SSMS returns the following message:
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(1 row(s) affected)
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This message is returned even if millions of rows were deleted because the message tells us only how many rows were deleted directly by the executed statement. There is nothing wrong with this behavior, but it is definitely something you should consider.
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If you have defined foreign keys with cascading actions, any AFTER triggers on the affected tables are still executed, but they are executed after the whole chain of cascading actions have completed. If an error occurs while the cascading action chain is being executed, the entire chain is rolled back and no AFTER triggers are executed for that chain.
Lesson 2: Declarative Data Integrity
CHECK CONSTRAINTS
Check constraints are a set of rules that must be validated prior to data being allowed into a table. Advantages to using check constraints include the following:
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They are simple to implement. (They are very similar to a WHERE clause.) They are checked automatically. They can improve performance.
A sample check constraint that verifies that a Product must have a non-negative price is shown here:
ALTER TABLE Products ADD CHECK(Price >= 0.0);
The simplicity of check constraints is a great advantage over using triggers. However, there are some disadvantages as well, such as the following:
Error messages from check constraints are system-generated and cannot be replaced by a more user-friendly error message. A check constraint cannot see the previous value of a column. This means that it cannot be used for some types of data integrity rules, such as Updates to the price column cannot increase or decrease the price by more than 10 percent.
One important aspect of check constraints is that they reject values that evaluate to False rather than accepting values that evaluate to True. That might seem like the same thing, but in SQL Server, it is not, because of an issue related to NULL values that is important to acknowledge. For example, if you have a check constraint that states that Price > 10.0, you can still insert a NULL value into the Price column. This value is allowed because any comparison made with NULL returns NULL it is neither True nor False. If you don t want the check constraint to allow the NULL value, you can either dissallow NULL in the Price column by specifying the NOT NULL constraint for the column or by changing the check constraint to read Price > 10.0 AND Price IS NOT NULL.
EXTENDING CHECK CONSTRAINTS WITH USER-DEFINED FUNCTIONS
User-defined functions (UDFs) created both in T-SQL and managed code (also referred to as .NET or CLR UDFs) can be an integral part of check constraints and are therefore discussed briefly here. They are covered in more detail in 5. The expression in a check constraint can contain most of the logic that you can use in a WHERE clause (including NOT, AND, and OR). It can call scalar UDFs and reference other columns in the same table; however, it is not allowed to contain subqueries directly. Because you can write your own scalar functions in either T-SQL or managed code, you can apply advanced logic inside your check constraints and, through them, even use subqueries. The following example creates a UDF called fnIsPhoneNumber in managed code (shown in both Microsoft Visual Basic and C#) to verify that a string contains a valid U.S. phone number by applying a regular expression:
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