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All tables should have one column or a combination of columns that uniquely identifies rows in the table. This is called the primary key, and it is covered in Lesson 2, Declarative Data Integrity, later in this chapter. Most of the time, it is difficult to select a column of data whose values make a good primary key, typically because values are not guaranteed to be unique or because the values might change frequently. Instead of using such a column, called a natural key, you can use a technical or automatically generated key. In SQL Server,
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Tables, Data Types, and Declarative Data Integrity
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the IDENTITY property is used to designate one column per table whose value should be automatically increased or decreased as new rows are added. The syntax for creating an identity column is <column name> <data type> IDENTITY(<seed>, <increment>) NOT NULL. The seed is the starting point for generating numbers, and the increment is the value by which the key is incremented (or decremented, if negative). An identity column cannot allow NULL values. The following example creates the HR.Employees table and defines the EmployeeId column with an identity that starts at 1000 and increments by a value of 2 for each row:
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CREATE TABLE HR.Employees ( EmployeeId INT IDENTITY(1000, 2) NOT NULL ,FirstName NVARCHAR(50) NOT NULL ,LastName NVARCHAR(50) NOT NULL ,BirthDate DATE NOT NULL );
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Note that the IDENTITY property can be specified only when creating a new column. An existing column cannot be modified to use the IDENTITY property. To change an existing column, the existing column must be dropped and the identity column added in its place. The identity column can be used only on data types that store whole numbers, which include both the integer data types and the decimal data type with a scale set to 0. Another important point to note with identity columns is that they are not guaranteed to generate complete sequences. If an insert fails, it still uses the identity value, creating a hole in the sequence. Consider this example:
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INSERT HR.Employees (FirstName, LastName, BirthDate) VALUES ('John', 'Kane', '1970-01-30'); -- EmployeeID generated: 1000 GO
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INSERT HR.Employees (FirstName, LastName, BirthDate) VALUES ('John', 'Kane', '1970-01-32'); -- Fails because of invalid date GO
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INSERT HR.Employees (FirstName, LastName, BirthDate) VALUES ('Jane', 'Dow', '1972-03-30'); -- EmployeeID generated: 1004 (1002 is missed) GO
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In this example, the second INSERT fails and the key 1002 is skipped, so Jane Dow is inserted with the key 1004.
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Lesson 1: Working with Tables and Data Types
Compression
SQL Server 2008 introduces the possibility of compressing the data in tables and indexes if you use SQL Server 2008 Enterprise Edition. Data compression is implemented in two levels: row and page. The following statement configures a table to use page-level compression:
ALTER TABLE HR.Employees REBUILD WITH (DATA_COMPRESSION = PAGE);
If you turn on row-level compression, SQL Server changes the format used to store rows. In simple terms, this row format converts all data types to variable-length data types. It also uses no storage space to store NULL values. The more fixed-length data types (such as datetime2, int, decimal, and nchar) that you use in a table, the more likely you are to benefit from row-level compression. Page-level compression includes row-level compression and adds page-level compression using page dictionary and column prefixing. Page dictionary simply introduces pointers between rows in the same page to avoid storing redundant data. Consider the following simplified page storing names:
Row 01: John Kane Row 02: John Woods Row 03: John Kane
If this page used page dictionary, it would look like this:
Row 01: John Kane Row 02: John Woods Row 03: 01
Here, the value in Row 03 points to the value in Row 01, saving several bytes in storage. Page compression also includes column prefixing, which is similar to page dictionary but can reuse parts of values. When considering whether to use row- or page-level compression, it is very important to verify the amount of space actually saved by turning on the compression.
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