Java Data Objects

Relational Modeling Constructs

Before we discuss the mapping between Java classes and a relational schema, we will first provide a brief summary of the modeling constructs found in relational schemas. This is not meant to cover all aspects of a relational schema; it will simply define the terms we use in this chapter.

A relational schema is organized as a set of tables. A table is usually defined for each entity in the application domain you are modeling. When you design an object model, an entity is represented by a class. Each table consists of rows and columns. A row contains the data for a specific instance of an entity being modeled. A column contains the values for one of the attributes of the entity. A table cell is the intersection of a particular row and column in the table, and it contains the value of an attribute for a specific entity instance.

The type of a column is the same for all rows of a table. Relational databases do not support Java’s capability for a field to reference one of many different types. ANSI SQL 92 defines a standard set of supported column datatypes. Relational database products support these standard datatypes and usually support their own additional, proprietary datatypes. One issue developers often contend with is the use of a datatype that is specific to one database product but not supported by another. JDO helps insulate your applications from these datatype differences, since you only deal with Java types, which are then mapped to the various ...