barcode generator in vb.net code project Creating and Altering Tables in Software

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Creating and Altering Tables
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In the second line of the syntax, you have the option to provide a default value (see the Specify Column Default Values section), column constraints (see 4), or a collation (see 2). At its most basic, a CREATE TABLE statement might look something like the following statement:
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CREATE TABLE ARTISTS ( ARTIST_ID INTEGER, ARTIST_NAME CHARACTER(60) );
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In this statement, I m creating a table named ARTISTS, a column named ARTIST_ID, and a column named ARTIST_NAME. The ARTIST_ID column is associated with the INTEGER data type, and the ARTIST_NAME column is associated with the CHARACTER data type. Notice that the two column definitions are separated by a comma. Also notice that I have placed the two column definitions on separate lines and aligned the data types by adding extra spaces all of this is to improve readability, but is otherwise unnecessary (when SQL statements are processed, extra spaces and new lines are simply ignored). If you execute the CREATE TABLE statement, your table will look similar to the table shown in Figure 3-1.
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The rows of data shown would not be in a table until you have actually added that data. The rows are shown here merely to give you an idea of the type of table that this statement would create.
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Before we go any further with the discussion of creating a table, let s take a closer look at data types, which play an integral role in any column definition.
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ARTIST_ID: INTEGER 10001 10002 10003 10004 10005 10006 10007 10008 10009 ARTIST_NAME: CHARACTER(60) Jennifer Warnes Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline Jose Carreras Placido Domingo Luciano Pavarotti
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Figure 3-1
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The ARTIST_ID and ARTIST_NAME columns of the ARTISTS table
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SQL: A Beginner s Guide
Ask the Expert
Q: A:
When you discussed the various types of tables that SQL supports, you talked briefly about temporary tables. What is the purpose of temporary tables Temporary tables provide you with a way to store temporary results within the context of your session. You might find that you need a place to store data in order to take a certain course of action. You can explicitly create a persistent base table, store data in it, and then drop the table when you re finished, but the temporary table allows you to do the same without having to explicitly destroy the table each time you use it. In other words, the temporary table is a useful tool when you need to store data for only a specific period of time. For example, suppose you have an application that allows you to generate a quarterly report based on your inventory at the end of the reporting period. The application might need to gather the data into a meaningful collection to generate the report; however, once the report is generated, the application no longer needs to store that data, so the table can be deleted. One of the advantages of using a temporary table is that, because it is unique to a session, the table cannot interact with other users or sessions. As a result, the RDBMS doesn t have to take special steps to lock the data to prevent other users from applying conflicting updates to the temporary tables, and bypassing locking can result in better performance.
Specify Column Data Types
Whenever you define a column in a CREATE TABLE statement, you must, at the very least, provide a name for the column and an associated data type or domain. The data type or domain (discussed in 4) restricts the values that can be entered into that column. For example, some data types limit a column s values to numbers, while other data types allow any character to be entered. SQL supports three types of data types:
Predefined Predefined data types are the most common. Each predefined data type is a named element (using an SQL keyword) that limits values to the restrictions defined by that database. SQL includes five types of predefined data types: string, numeric, datetime, interval, and Boolean. Constructed Constructed data types are also a named element but tend to be more complex than predefined data types because they can hold multiple values. Constructed types allow you to construct more complicated structures than more traditional data types. A thorough discussion of these types is beyond the scope of this book, but I wanted to mention them so you know that they exist.
3:
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