PART 6 ■ ■ ■ Appendixes APPENDIX A ■ ■ ■ Glossary This appendix contains a brief description of the most important notions from the world of data- bases and related areas. Character Set: A character set is a specification that determines the set of codes used to repre- sent the characters of a particular language or set of languages. Among the oldest character sets is the ASCII (American Standard Code for Information Interchange) character set, in which the letter A is coded by the integer 65. Client: A client is a program that accesses a central service. In connection with databases, a client denotes a program that accesses a central database. The database is managed by another program, the database server. Client/Server Architecture: Most modern database systems are based on a server that man- ages the data and executes SQL commands, and an arbitrary number of clients that access the server over a network. This concept is called client/server architecture. Alternatives to this model are file-server databases such as dBase and Access, in which all clients access the data- base file directly through a common file system. (This is quite inefficient when many users are trying to edit data simultaneously.) Cluster: In information theory a cluster usually refers to a group of networked computers that work together on a common task. A database cluster is therefore a database system in which several computers provide a common database (particularly a large or fast or secure one). Cursor: In MySQL a cursor enables step-by-step processing of query results (particularly in stored procedures). In the ADO library, cursor properties determine what processing options exist for the records of a Recordset object. Data Dictionary: The MySQL Data Dictionary enables one to use SELECT queries to obtain information about the structure of databases and tables as well as other administrative data about the MySQL server. The MySQL server provides, since version 5.0, virtual INFORMATION_ SCHEMA tables. Data Record: A data record, or simply record, is a row of a table. Database: Generally speaking, a database is an ordered collection of data. In connection with MySQL, a database consists of a number of tables. In common usage, a database can also refer to an entire database system, that is, the program that manages the data. In MySQL, this pro- gram is called the database server (MySQL server). Database Server: A database server is a program that permits access to databases over a net- work. The server is responsible for the management of the data and for the execution of SQL commands. 723 724 APPENDIX A ■ GLOSSARY Derived Tables: These are tables that do not exist as such but are merely the result of a SELECT command. Derived tables are in fact nothing more than a variant of subSELECTs, that is, nested SELECT commands. Domain Name: The domain name denotes an entire network. A local network might consist of computers with the names mars.sol, uranus.sol, and jupiter.sol, in which case mars, uranus, and jupiter are computer names (hostnames), and sol is the domain name. Domain Name Server (DNS): Internally, communication within a network takes place on the basis of IP numbers (e.g., 192.168.0.27). Since mere mortals do not keep track of such numbers very well, each computer also has a computer name, composed of host name and domain name (e.g., uranus.sol). Whenever such a name is used, the running program must determine the associated IP num- ber. Sometimes, the converse case arises, and for a given IP number, the computer name is required. In both situations, the domain name server provides the answer. (Small networks can get by without a DNS. In that case, the association between IP numbers and computer names is determined from static tables, under Unix/Linux in the file /etc/hosts.) Foreign Key: A foreign key is a unique value (usually an integer) that refers to a particular col- umn of another table. A foreign key thus creates a link (relation) between two tables. Full-Text Index: A full-text index enables the search for several words in arbitrary order within a text. To make full-text search efficient, a special index is required, known as a full-text index. Global Assembly Cache (GAC): This notion is specific to Microsoft. It describes a central direc- tory of all *.dll files for .NET programs. In contrast to the Windows system directory, in the GAC, several different versions of a library can be installed simultaneously without leading to a conflict. Host Name: The host name is the name of a computer valid over a network. If the complete name of the computer is jupiter.sol, then jupiter is the host name and sol the domain name. Hot Backup: What makes a hot backup hot is that during the backup process, the database server can continue to run almost completely undisturbed. (During a normal backup, on the other hand, no changes in the data may be made.) Hot backups can currently be made in MySQL only with a commercial auxiliary program that works only for InnoDB tables. Include File: An include file is a part of program code. It is read by the compiler (e.g., C) or during code execution (e.g., PHP). Index: In database-speak, an index, or its synonym key, is an ordered list of the content of a table. An index serves primarily to speed up access to individual records: Instead of searching through all records, it suffices to search the index. Then, the desired record can be read directly. The use of an index is comparable to that of an index in a book. InnoDB Tables: In MySQL, tables can be stored on the hard drive in more than one format, including MyISAM and InnoDB. InnoDB tables possess some additional functionality over MyISAM tables, such as transactions and foreign key constraints. The name InnoDB is derived from the name of the company Innobase, with DB being short for database. Internet Service Provider (ISP): An ISP is a company that provides technical support and host- ing of web sites. ISPs are used by commercial enterprises and private individuals who wish to maintain web sites on the Internet but do not themselves possess a permanent or sufficiently high-speed Internet connection. Be careful in your choice of ISP that it supports the programs presented in this book (Apache, PHP, MySQL, etc.). There are, of course, ISPs that offer Microsoft products instead. Key: A key is an ordered index of a table. See also the entry of the synonym Index. APPENDIX A ■ GLOSSARY 725 Logging: Logging refers to the recording of SQL commands in a file. The motivation for logging is usually to have available all the changes to the database made since the last backup so that the database can be re-created in the event of disaster. With MySQL, logging is also a prerequi- site for replication. MyISAM Tables: The MySQL server can store tables in various formats. The most important such format is called MyISAM, which stands for indexed sequential access method, referring to the method of access to data records within a file with the aid of an index. An alternative to MyISAM tables is InnoDB tables. Named Pipes: Named pipes represent a mechanism whereby two programs can communicate under Windows 2000/XP. Data exchange takes place according to the principle of first in, first out (FIFO). Normal Form: A database generally consists of a number of tables. A goal in the design of tables is to avoid redundancy. This goal is supported by particular rules. When the first n such rules are adhered to, the database is said to be in nth normal form. Normalization: This term refers to the optimization of the database design to avoid redundancy. The goal is generally to achieve third normal form. PHP: PHP is a script language for programming dynamic web sites. The program code is located in *.php files. These files are executed by the PHP interpreter at each access over the web server. The resulting HTML document is passed on to the web server. Port: Network packets that are sent using the TCP/IP protocol are always addressed to a partic- ular port. The port number is thus part of the address to which the packet is sent. Ports help in categorizing network traffic. For example, for data exchange between web browsers and web servers, port 80 is used. MySQL uses port 3306 by default. Prepared Statements: These provide for a particularly efficient processing of SQL commands that differ only in the values of parameters. The entire SQL command is sent only once to the MySQL server, and thereafter only the varying parameters. This not only reduces the quantity of data that has to be transferred, but also spares the server the repeated analysis of the SQL command. Primary Index (Primary Key): The primary index identifies each record of a table uniquely. Normally, the primary key field is an integer column. Whenever a new record is inserted, the database server determines a new, unique value for this column. Privilege: In MySQL nomenclature, a privilege is a right to execute particular database opera- tions (e.g., read or change data in a particular table). Query: In database language, a query is generally a SELECT command. The syntax of this com- mand is part of the language SQL (structured query language). SQL specifies how commands to the database server must be structured. Query Cache: In many database applications, it happens that the same queries must be exe- cuted over and over. To speed up such queries, MySQL possesses, since version 4.0, a query cache, that is, temporary storage in which queries and their results can be stored.