Creates a relation definition. A relation definition consists
    of a list of fields that make up an Oracle Rdb record. When the
    DEFINE RELATION statement executes, Oracle Rdb adds any constraints
    associated with the relation definition to the physical database.

    Example:

    RDO> DEFINE RELATION DEPARTMENTS.
    cont>    DEPARTMENT_CODE
    cont>      PRIMARY KEY.
    cont>    DEPARTMENT_NAME.
    cont>    MANAGER_ID
    cont>       BASED ON ID_NUMBER.
    cont>    BUDGET_PROJECTED
    cont>       BASED ON BUDGET.
    cont>    BUDGET_ACTUAL
    cont>       BASED ON BUDGET.
    cont> END DEPARTMENTS RELATION.