free barcode generator source code in vb.net The System Catalog in Software

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The System Catalog
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the database administrator may restrict system catalog access to provide an additional measure of database security. By querying the system catalogs, you can discover information about the structure of a database, even if you have never used it before. User access to the system catalog is read-only. The DBMS prevents users from directly updating or modifying the system tables because such modifications would destroy the integrity of the database. Instead, the DBMS itself takes care of inserting, deleting, and updating rows of the system tables as it modifies the structure of a database. Data Definition Language (DDL) statements such as CREATE, ALTER, DROP, GRANT, and REVOKE produce changes in the system tables as a byproduct of their actions. In some DBMS products, even DML statements that modify the database, such as INSERT and DELETE, may produce changes in the system tables, which keep track of how many rows are in each table.
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The Catalog and Query Tools
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One of the most important benefits of the system catalog is that it makes possible user-friendly query tools, as shown in Figure 16-1. The objective of such tools is to let users simply and transparently access the database without learning the SQL language. Typically, a tool leads the user through a series of steps like this one: 1. The user gives a name and password for database access. 2. The query tool displays a list of available tables. 3. The user chooses a table, causing the query tool to display a list of the columns it contains. 4. The user chooses columns of interest, perhaps by clicking their names as they appear on a PC screen. 5. The user chooses columns from other tables or restricts the data to be retrieved with a search condition. 6. The query tool retrieves the requested data and displays it on the user s screen. A general-purpose query tool like the one in Figure 16-1 will be used by many different users, and it will be used to access many different databases. The tool cannot possibly know in advance the structure of the database that it will access during any given session. Thus, it must be able to dynamically learn about the tables and columns of a database. The tool uses system catalog queries for this purpose.
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DATABASE STRUCTURE
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The Catalog and the ANSI/ISO Standard
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The ANSI/ISO SQL1 standard did not specify the structure and contents of the system catalog. In fact, the SQL1 standard does not require a system catalog at all. However, all of the major SQL-based DBMS products provide a system catalog in one form or another. The structure of the catalog and the tables it contains vary considerably from one brand of DBMS to another.
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SQL: The Complete Reference
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Figure 16-1.
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A user-friendly query tool
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Because of the growing importance of general-purpose database tools that must access the system catalog, the SQL2 standard includes a specification of a set of views that provide standardized access to information typically found in the system catalog. A DBMS system that conforms to the SQL2 standard must support these views, which are collectively called the INFORMATION_SCHEMA. Because this schema is more complex than the actual system catalogs used by commercial DBMS products, and is only slowly being supported, it is described in a separate section near the end of this chapter titled The SQL2 Information Schema.
Catalog Contents
Each table in the system catalog contains information about a single kind of structural element in the database. Although the details vary, almost all commercial SQL products include system tables that describe each of these five entities: I Tables. The catalog describes each table in the database, identifying its table name, its owner, the number of columns it contains, its size, and so on.
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