4-5 Relational Database

Relational database is the most popular model for GIS. For example, the following relational database softwares are widely used.

- INFO in ARC/INFO
- DBASE III for several PC-based GIS
- ORACLE for several GIS uses

Relational database is a model to link the complex spatial relationships between objects.

The spatial objects are tabulated in tables consisting of records with a set of attributes as shown already in Figure 4.3 (c). Each table (called relation) consists of different number of attributes, which is called the degree. The degree of an attributes refers to n-ary (ex. unary, binary etc.) relation.

In a relational model, the following two important concepts should be defined.

Key of relation ; a subset of attributes
Unique identification ; e.g. the key attributes is a phone directory in a set of last name, first name and address.

non redundancy ; any key attribute selected and tabulated should keep the key's uniqueness. e.g. address can not be dropped from telephone address, because there may be many with the same names.

Prime attribute : an atribute listed in at least one key.
The most important point of the relational database design is to build a set of key attributes with a prime attribute, so as to allow dependence between attributes as well as to avoid loss of general information when records are inserted or deleted.

Table 4.2 shows how to build the relational database by normalizing an unstructured table.

The advantages of the relational database are as follows.

- there is no redundancy.
- type of building of an owner can be changed without destroying the relation between type and rate.
- a new type of building for example "Clay" can be inserted.