In a hierarchical database europe email list, fields or records are arranged in related groups resembling a family tree, which child records subordinate to parent records. The parent record at the top of the database is called the root record. Used principally on mainframes, hierarchical databases are europe email list the oldest of the four types. They are still used in some types passenger reservation systems. In hierarchical databases, accessing or updating data is very fast, because the relationship have been predefined. However because the structure must europe email list be defined in advance, it is quite rigid. There may be only one parent per child, and no relationships among the child records are possible. Moreover, adding new fields to databases, records requires that the entire europe email list database be redefined.

Also used principally with mainframes the network database is more flexible than the hierarchical database arrangement, because different relationships may be established between branches of data europe email list. However it still requires that the structure be defined in advance. Moreover, these are limits to the number of possible links among records. More flexible then both the databases discussed above, the relational database relates, or connects data in europe email list different files through the use of a key field, or common data element. In this arrangement europe email list there are no access paths down through a hierarchy. Instead data elements are stored in different tables made up of rows and columns. In database terminology, the tables are called relations (files), the rows are called europe email list tuples (records) and the columns are called attributes (fields).

The advantage of relational europe email list databases is that the user does not have to be aware of any structure. Thus, they can be used with a little training. Moreover, entries can easily be added, deleted or modified. A disadvantage is that some searches can be time-consuming europe email list. Nevertheless, the relational model has become popular for microcomputer database management programs, such as Paradox and Access.