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an overriding need to guarantee uniqueness of a row within the entire universe of tables that are ever created that you are willing to consume 32 bytes of memory for every join operation that you must perform
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What are the six categories of standard data types that you can use to define columns in tables, and what is the general purpose of each category The exact numeric data types store precise integer or decimal values; approximate numeric data types store floating-point numbers; monetary data types are used to store currency accurate to four decimal places; datetime data types are used to store dates and times; character data types are used to store text values; and binary data types are used to store binary streams, normally files.
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The second characteristic of any column definition is whether it requires a value to be stored. Databases have a special construct called a null that can be used. A null is not a value, nor does it consume storage. A null denotes the absence of a value and is used to rep resent something similar to unknown or not applicable. The best way to under stand this construct is to look at an example. You are designing a table to store addresses for your company s customers. You have decided that each address can have up to three address lines, a city, a state or prov ince, a postal code, and a country. So you create a table that contains seven col umns. Not every customer needs all three address lines to capture the street address, so one or two of these columns are not necessary for some addresses. Some customers live in countries that do not have states or provinces, so this column is also not necessary for every customer. When users input an address, they might not know the postal code of the customer, but they still need to be able to save all the data that is known. So do you stick a dummy value into the columns that either don t have values or the values aren t known when the data was entered Instead of putting dummy data (which is actually invalid data that might be seen by a cus tomer) into a table, the data is generally omitted. Because the data was not explicitly
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Lesson 1: Designing Database Tables
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specified, it is either unknown or not applicable. In the database, the column would be null to designate this unknown state. Because it is impossible for the absence of something to equal the absence of something, a null cannot be used in comparisons because a null does not equal another null.
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MORE INFO
Nulls
For details about nulls, please refer to the Books Online article, Null Values, at ms-help:// MS.SQLCC.v9/MS.SQLSVR.v9.en/udb9/html/f77b3bfe-b5be-417e-96a3-ff3a19c33119.htm.
When you define columns, you can specify whether nulls are allowed. If you disallow nulls, a user is required to specify a value for the column. One of the biggest mistakes that database professionals make is to omit the nullability definition in the CREATE TABLE statement. By doing so, they trust that the value that is set as the default is suf ficient for their needs. All someone needs to do is change the default nullability for columns, and the applications will suddenly become broken.
Identity
A special property that you can specify for a single column in a table is an identity. The data types that you can use with the identity property are bigint, int, smallint, tinyint, decimal, and numeric, all of which are exact numeric data types. If you use decimal and numeric, you must define the columns with zero decimal places. Defining a column with the identity property causes SQL Server to generate an auto matically incrementing number. The identity property takes two parameters: seed and increment. The seed value designates the starting value that SQL Server uses; the increment value specifies what number SQL Server adds when generating each suc cessive value. They are equivalent to autonumber or autoincrement values in other languages.
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