Creating Tables, Constraints, and User-Defined Types in .NET

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Creating Tables, Constraints, and User-Defined Types
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You use constraints to enforce additional business rules within a table. You can use constraints to ensure that duplicate values cannot be entered into a column or that a column can allow only values that meet a specified condition. You can use constraints to enforce complex pattern matching such as the Vehicle Identification Number (VIN) that is used to uniquely identify every vehicle. You can also create constraints to ensure that a value cannot be entered in one table unless it already exists in another table, for example, not allowing an address to be entered unless a customer already exists for the address.
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The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
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Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
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1. Which of the following objects can you use in a check constraint (Choose all that apply.) A. System function B. Stored procedure C. User-defined function (UDF) D. View
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Lesson 3: Creating User-Defined Types
Lesson 3: Creating User-Defined Types
User-defined types (UDTs) have two purposes in SQL Server 2005. You can use Transact-SQL-based UDTs to enforce consistency in table definitions, and you can use Common Language Runtime (CLR) UDTs to create new data types that do not exist in SQL Server. In this lesson, you see how and when to create each type of UDT.
After this lesson, you will be able to:
Explain the differences between Transact-SQL and CLR UDTs. Create a Transact-SQL UDT. Create a CLR UDT.
Estimated lesson time: 20 minutes
Transact-SQL UDTs
You use Transact-SQL UDTs essentially as an aliasing mechanism to provide consistency in table definitions within a database. For example, you might have customers, vendors, manufacturers, and employees stored in the same database. Because of differences in the data that you store for each entity, you might have separate address tables for each one. Even though you have four different address tables, a City column exists in each one. The City column holds variable-length character data with a maximum size of 30 characters. You could implement the City column as a varchar(30) in each table, or you could use a Transact-SQL UDT to ensure that all City columns are defined the same. To create a UDT, you use the CREATE TYPE command as follows:
CREATE TYPE [ schema_name. ] type_name { FROM base_type [ ( precision [ , scale ] ) ] [ NULL | NOT NULL ] | EXTERNAL NAME assembly_name [ .class_name ] } [ ; ]
The following command would create a UDT for the City column discussed previously:
CREATE TYPE udt_city FROM varchar(30) NOT NULL ;
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Creating Tables, Constraints, and User-Defined Types
You could then use this UDT when you are defining a table, as follows:
CREATE TABLE dbo.CustomerAddress (CustomerAddressID int AddressTypeID tinyint dbo.AddressType(AddressTypeID), PrimaryAddressFlag bit AddressLine1 varchar(30) AddressLine2 varchar(30) AddressLine3 varchar(30) City udt_city StateProvinceID int dbo.StateProvince(StateProvinceID), PostalCode char(10) CountryID int dbo.Country(CountryID)); IDENTITY(1,1) PRIMARY KEY CLUSTERED, NOT NULL FOREIGN KEY (AddressTypeID) REFERENCES NOT NULL, NOT NULL, NULL, NULL, NOT NULL, NULL FOREIGN KEY (StateProvinceID) REFERENCES NULL, NULL FOREIGN KEY (CountryID) REFERENCES
Transact-SQL UDTs are always created using base data types.
CLR UDTs
You can use the CLR integration in SQL Server 2005 to create your own data types beyond those that already exist within SQL Server.
Defining New Data Types
There is at least one person reading this whose brain shifted into overdrive when they read that you can create new data types in SQL Server 2005. So before we get started on this topic, we need to do a serious reality check. First, you cannot create CLR UDTs unless you turn on CLR capability by using the Surface Area Configuration utility. Second, SQL Server is NOT an object database. Thousands of developers have invested tens of thousands of man-years of development in SQL Server. The same goes for Oracle, DB2, and Sybase. Together, these four database management systems (DBMSs) represent nearly all the database market. And none of these DBMSs has support for object data types. That doesn t mean that someone can t come up with a way to do create such data types; it simply means that if an actual market existed for such things, at least one of the vendors would have written it by now or at least licensed an implementation from someone and added into their product. Before you go out and spend several hundred hours creating the Person data type or the Customer data type, carefully consider exactly what you are imposing on your system. CLR data types must be written to a specification that places 15 very stringent requirements on the interfaces and code specification. You must also create
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