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THE UBIQUITOUS DATABASE Today databases are ubiquitous. Almost every application we encounter has a database foundation. When we buy something on-line, when we renew our driver s license, when we inquire about a flight schedule, when we look up the sports scores, we are using applications that rely on databases. Databases provide efficiency, security and flexibility of data storage, and are employed in applications ranging from library card catalogs to machine automation in factories. This was not always so. Soon after computers entered the second generation of the modern era (i.e., the late 1950s), the availability of high-level programming languages and large storage capacities (usually magnetic tape) led to larger and larger collections of data. The data were stored in files collections of data records and it soon became clear that this approach presented a number of difficulties. First, larger files took longer to search. Recall from our discussion of algorithms that a sequential search operates in O(n) time. Therefore, the larger the file, the more time a search for any particular item requires. That may not be a problem when you re keeping track of the birthdays of your friends, but if you re keeping a record of every MasterCard purchase transaction for millions of customers, the slow retrieval of information by serial search becomes prohibitive. Other problems appeared as well. For instance, if you store the billing address of the customer in each record of each sale, you waste a lot of space storing data redundantly. Suppose that a customer changes their address; you have to rewrite all the transactions on record, using the new address. You might decide to solve this problem by putting the billing addresses in a separate file. That would save space in the transaction file, but now in order to compute a customer s bill, you must search serially through two files. DATABASE TYPES Starting in the late 1960s, database systems were developed to deal with these and other problems. Two early types of databases were the hierarchical and networking types. IBM offered DL/1, a hierarchical database, and various other hardware and software companies offered networking databases on the CODASYL (Conference On DAta SYstems Language, Database Task Group) model. IDMS was a particularly successful CODASYL database. The hierarchical and networking database structures organized files together to provide more rapid access to the information, better security, and easier updates. However, the structures were complex, tied to the implementation details of the file system, and fairly rigid. In 1970, E. F. Codd (Edgar, Ted, an Englishman who moved to the US after serving in WWII) of IBM proposed the relational database model. The relational model relied heavily on mathematical theory. At the time, it may have sounded dreamy, as data were simply to be stored in tables (called relations ). Each relation/ table would maintain information about one entity type (type of thing), and entities would be related to one another by virtue of information stored in the tables, rather than by external pointers or other devices.
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Codd also proposed a language for data access that was based on set theory (in the 1980s IBM would bring structured query language (SQL) to the world). To many professionals at the time, Codd s proposals seemed impossible to implement efficiently. However, in the 1980s, Oracle became the first company to offer a commercial implementation of a relational database, and IBM began selling its relational database called DB2. Today the relational data model is predominant, and that is the model on which this chapter will focus. With the advent of object-oriented programming, new data management designs called object-relational or object-oriented database management have been developed. These systems promise convenience and congruity of operation with OO programming techniques. While they have not yet become widely successful, they may become so in the future. ADVANTAGES OF USING A DATABASE The primary motivation for using a database is speed of access. Assuming proper database design, access to individual pieces of information can be essentially instantaneous, regardless of the number of data records or the size of the database. The experience of instantaneously finding exactly the record in which you are interested, from among millions, can be a stunning one. Access speed can be essentially zero and constant, regardless of n, the number of records. This can be expressed as O(k), where k is a constant of a small value. Such performance becomes possible because the database management system stores data about the data (metadata) as well as the data itself. Metadata also makes data stored in a database self-describing. This means that programs accessing the data don t need to know so many details regarding how the data are stored. If a program reads from an ordinary file, it must know about data types, formats, and the order of fields. However, when a program reads from a database, it often needs only specify what information it requires. A database also allows for efficient utilization of storage space. One of the consequences of good database design is that duplication of data is minimized. When mass storage devices were more expensive, this virtue was more important, but minimizing redundancy is still helpful in promoting efficiency, avoiding errors, and protecting against corruption of data. Database management systems (DBMS) also promote data security in a variety of ways. For instance, data backup and recovery facilities are always built into the DBMS, and data can be copied to a backup medium, even as the database continues to operate. Database systems also support the concept of a transaction. A transaction is a group of related changes to the data, where all changes must occur, or else none must occur. The familiar example is removing funds from a savings account and depositing those funds in a checking account. We want both the withdrawal and the deposit to succeed, but if the withdrawal succeeds and the deposit fails, we want the withdrawal to be rolled back and the money put back into the savings account. The two changes constitute a single transaction which must either succeed in its entirety, or be rolled back to have no effects whatsoever. Database systems allow changes to the data to be grouped into transactions that are either committed upon full success, or entirely rolled back upon any failure. DBMSs also promote data security by organizing use by multiple users. Imagine an enterprise like Amazon.com where many users from all over the world interrogate the database of available titles, and place orders, simultaneously. The DBMS coordinates multiuser access so as to preserve data integrity. Changes made by one user will not interfere with the use of the database by another. The DBMS manages potential conflicts by providing temporary locks on the data when necessary. For all these reasons, database systems have become ubiquitous. As we will see, the use of database systems has been facilitated, too, by a set of language standards called SQL. It is difficult to imagine any substantial application today that does not include a database, or provide a direct link to an existing database. MODELING THE DATA DOMAIN Before creating a relational database, the designer goes through a process called data modeling. The modeling phase identifies the entities which will be of interest, the attributes of each entity type, and the relationships between different entity types.
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