Computed Columns in .NET

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You can also create a special type of column called a computed column, which contains a computation involving one or more other columns in the table. By default, the computed column contains a definition for the computation but does not physically store data by default. When the data is returned, the computation is applied to return a result. However, you can force a computed column to physically store data by using the PERSISTED keyword. This keyword causes the computation to occur when the row is inserted or modified, and the result of the computation is then physically stored in the table.
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Lesson 1: Creating Tables
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Creating a Table
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Now that you have seen all the column details you can specify to define the structure of a table, you are ready to actually create a table. You can create three different types of tables in SQL Server: permanent, temporary, and table variables.
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Normalization, naming conventions, and table design
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Normalization, naming conventions, and various table-design methods are beyond the scope of this book. For information about these topics, see MCITP Self-Paced Training Kit (Exam 70-443): Designing a Database Server Infrastructure by Using Microsoft SQL Server 2005, Microsoft Press, 2007.
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Permanent Tables
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To create a table, you use the CREATE TABLE Transact-SQL command. The general syntax of this command is as follows:
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CREATE TABLE [ database_name . [ schema_name ] . | schema_name . ] table_name ( { <column_definition> | <computed_column_definition> } [ <table_constraint> ] [ ,...n ] ) [ ON { partition_scheme_name ( partition_column_name ) | filegroup | "default" } ] [ { TEXTIMAGE_ON { filegroup | "default" } ] [ ; ]
To execute this command, you must be a member of the sysadmin fixed server role, a member of the database owner fixed database role, or have been granted the CREATE TABLE permission. When you use this command, you create a table in the database that can be accessed by any user with the appropriate permissions. The ON clause specifies where the table will reside on physical storage. If you do not specify a filegroup, SQL Server creates the table on the default filegroup. Using our earlier example, you could use the CREATE TABLE command to create the CustomerAddress table as follows:
CREATE TABLE dbo.CustomerAddress (AddressLine1 varchar(30) AddressLine2 varchar(30) AddressLine3 varchar(30) City varchar(50) StateProvinceID int PostalCode char(10) CountryID int NOT NULL, NULL, NULL, NOT NULL, NULL, NULL, NULL)
3
Creating Tables, Constraints, and User-Defined Types
This table definition specifies the following:
The table will be created in the dbo schema. A minimum of one address line that has a maximum of 30 characters must be specified for every customer. The storage space consumed will be equal to the number of characters in the column. One or two optional address lines can be specified, each holding up to 30 characters and consuming storage space equal to the number of characters in the column. A customer record must have a city specified; the City column can hold a value up to 50 characters in length and consumes storage equal to the number of characters in the column. A customer can have an optional state/province specified. The column consumes four bytes of storage and contains an integer value. A customer can have an optional postal code specified. Each row consumes 10 bytes of storage. A customer can have an optional country specified. The column consumes four bytes of storage and contains an integer value.
Although the preceding table definition accurately captures the necessary data, you might have noticed a few problems. A customer might have one or more home addresses, one or more business addresses, and one or more shipping addresses. A customer might also want to designate a particular address as the primary address. So you might be tempted to add a lot of additional columns to handle these situations. But that would be thinking in terms of a spreadsheet, not a database. Instead, you can simply add a column to the table that designates the type of address and a column to designate the primary address, as the following example shows:
CREATE TABLE dbo.CustomerAddress (AddressType char(4) PrimaryAddressFlag bit AddressLine1 varchar(30) AddressLine2 varchar(30) AddressLine3 varchar(30) City varchar(50) StateProvinceID int PostalCode char(10) CountryID int NOT NULL, NOT NULL, NOT NULL, NULL, NULL, NOT NULL, NULL, NULL, NULL)
For now, we will ignore the questions concerning the StateProvinceID and CountryID columns because we will cover them in the next lesson on constraints.
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