The relational database concept was described first by Dr. Edgar F. Codd in an IBM research publication entitled "System R4 Relational" appearing in 1970. Initially, it was unclear whether any system based on this concept could achieve commercial success. Nevertheless, Relational Software, Incorporated (RSI) began in 1977 and released Oracle V.2 as the world's first relational database within a couple of years. By 1985, Oracle could claim more than 1,000 relational database customer sites. By comparison, IBM would not embrace relational technology in a commercial product until the Query Management Facility in 1983.
Why has relational database technology grown to become the de facto database technology since that time? A look back at previous database technology may help to explain this phenomenon.
Database management systems were first defined in the 1960s to provide a common organizational framework for what had been data stored in independent files. In 1964, Charles Bachman of General Electric proposed a network model with data records linked together, forming intersecting sets of data, as shown on the left in Figure 1-1. This work formed the basis of the CODASYL Data Base Task Group. Meanwhile, the North American Aviation's Space Division and IBM developed a second approach based on a hierarchical model in 1965. In this model, data is represented as tree structures in a hierarchy of records, as shown on the right in Figure 1-1. IBM's product based on this model was brought to market in 1969 as the Information Management System (IMS). As recently as 1980, almost all database implementations used either the network or hierarchical approach. Although several competitors utilized these technologies, only IMS remains.
The relational database uses the concept of linked two-dimensional tables consisting of rows and columns, as shown in Figure 1-2. Unlike the hierarchical approach, no predetermined relationship exists between distinct tables. This means that the data needed to link together the different areas of the network or hierarchical model need not be defined. Because relational users don't need to understand the representation of data in storage to retrieve it (many such users created ad hoc queries against the data), ease of use helped popularize the relational model.
Relational programming is nonprocedural and operates on a set of rows at a time. In a master-detail relationship between tables, there can be one or many detail rows for each individual master row, yet the statements used to access, insert, or modify the data would simply describe the set of results. In many early relational databases, data access required the use of procedural languages that worked one record at a time. Because of this set orientation, programs can access more than one record in a relational database more easily. Relational databases can be used more productively to extract value from large groups of data.
The contents of the rows in Figure 1-2 are sometimes referred to as records. A column within a row is referred to as a field. Tables are stored in a database schema, which is a logical organizational unit within the database. Other logical structures in the schema often include the following:
Views
Provide a single view of data derived from one or more tables or views. The view is an alternative interface to the data, which is stored in the underlying table(s) that make up the view.
Sequences
Provide unique numbers for column values.
Stored procedures
Contain logical modules that can be called from programs.
Synonyms
Provide alternative names for database objects.
Indexes
Provide faster access to table rows.
Database links
Provide links between distributed databases.
The relationships between columns in different tables are typically described through the use of keys, which are implemented through referential integrity constraints and their supporting indexes. For example, in Figure 1-2, you can establish a link between the DEPTNO column in the second table, which is called a foreign key, to the DEPTNO column in the first table, which is referred to as the primary key of that table.
Finally, even if you define many different indexes for a table, you don't have to understand them or manage the data they contain. Oracle includes a query optimizer (described in Chapter 4) that chooses the best way to use your indexes to access the data for any particular query.
The relational approach lent itself to the Structured Query Language (SQL). SQL was initially defined over a period of years by IBM Research, but it was Oracle Corporation that first introduced it to the market in 1979. SQL was noteworthy at the time for being the only language needed to use relational databases, because you could use SQL:
For queries (using a SELECT statement)
As a Data Manipulation Language or DML (using INSERT, UPDATE, and DELETE statements)
As a Data Definition Language or DDL (using CREATE or DROP statements when adding or deleting tables)
To set privileges for users or groups (using GRANT or REVOKE statements)
Today, SQL contains many extensions with ANSI/ISO standards that define its basic syntax.
In 1983, RSI was renamed Oracle Corporation to avoid confusion with a competitor named RTI. At this time, the developers made a critical decision to create a portable version of Oracle (Version 3) that ran not only on Digital VAX/VMS systems, but also on Unix and other platforms. By 1985, Oracle claimed the ability to run on more than 30 platforms. Some of these platforms are historical curiosities today, but others remain in use. (In addition to VMS, early operating systems supported by Oracle included IBM MVS, DEC Ultrix, HP/UX, IBM AIX, and Sun's Solaris version of Unix.) Oracle was able to leverage and accelerate the growth of minicomputers and Unix servers in the 1980s. Today, Oracle is portable to both Microsoft Windows and Linux, which are the leading operating systems on popular commodity servers.
In addition to multiple platform support, other core Oracle messages from the mid-1980s still ring true today, including complementary software development and decision support tools, ANSI standard SQL across platforms, and connectivity over standard networks. Since the mid-1980s, the database deployment model has evolved from dedicated database application servers to client/server to Internet computing implemented with PCs and thin clients accessing database applications via browsersand, to the grid with Oracle Database 10g.
Oracle introduced many innovative technical features to the database as computing and deployment models changed (from offering the first distributed database to the first Java Virtual Machine in the core database engine). Oracle also continues to support emerging standards such as XML and .NET. Table 1-1 presents a short list of Oracle's major feature introductions.