Duplicating data across tables

Looking back at the four normalized tables (Table 1–Table 4), you may notice that there is some duplication of data across them. For example, the passenger number attribute is held in both the passenger table and the seat table. Seat number is held in the seat table and the aircraft table, and there are more duplications as well.

This duplication occurs in normalized tables because they are generally being developed to be used in a relational database. Relational systems use these common attributes as links between one table and the next. The links provide paths around the database so that every table can access the data held in every other table. This means that, apart from the linking attributes, no data needs to be duplicated in the database.

In the example hierarchical database discussed in this information, links of this kind are not needed because the tables are joined by pointers. Pointers provide a means of linking one table or file with another. Use the DBIDX and DBDIX z/TPFDF macros to maintain pointers.