com, 2003). Like these, there are many more operational database programs available on the Web. Next, let's talk about distributed databases. It is a collection of several different databases that looks like a single database to the user. These databases can be found on network servers on the World Wide Web, or on corporate intranets or extranets (O'Brien, 2003). An example of a distributed database is the Internet Domain Name System (DNS) (foldoc.doc.ic.ac.uk, 2003). .
Finally, we will talk about external databases. These are research related databases that can be found on the World Wide Web. Data, such as statistics, abstracts, copies of newspapers, magazines, newsletters, research papers, and many other types of data are found in external databases all over the Web, either for a fee from online services, or even without a charge from many sources on the Internet (O'Brien, 2003). Now that we have discussed the three main different types of databases, we can move on to various database structures. There are basically five of them. We will start with the hierarchical structured database. This is a multilevel structure, which consists of one root record and a number of subordinate levels. This makes it a "one-to-many"" relationship among the records, because each piece of data is related to the one element above it. The piece of data located at the top of the tree is called the root element, which is ironic considering that the root of a tree is actually at the bottom (O'Brien, 2003). The hierarchical structured database system was created in the 1960s. It was intended for use with commercial data, which were also generally hierarchical in structure (btinternet.com, 2003). Most early mainframe database management system packages used this type of structure (O'Brien, 2003).
The hierarchical structure was deemed insufficient in the late 60s because real connections between certain items of data were found to cut across branches of the pre-defined trees, therefore, the network structure emerged (btinternet.