There are several kinds of databases, the common types being:

• Hierarchical;
• Network;
• Relational;
• Object;
• Object-Relational.

Among these, the hierarchical and network database types are of older design than relational databases, but the others are newer.

An hierarchical database is built upon record types and a parent-child relationship among record types. All accesses to child records must go through parent records. Relationships are strictly hierarchical, and every record type must be a child only to a single parent. This makes it difficult to represent data where loops or other cross-links exist in relationships. Information storage and retrieval are fast, but querying is not, and it is not possible to build a decision support system, using this database model.

Under the network model, a more realistic relationship is assumed between data elements--a record type can be linked to more than one other record type. The paths needed to...