The extended table of a given Base Table is a virtual table composed of the base table itself plus all the tables that have N-1 relationships, either directly or indirectly, starting from the given base table.
For example, consider the following Bachman Diagram:
- The INVOICE base table will have the following extended table: {INVOICE, CUSTOMER, COUNTRY}
- The CUSTOMER base table will have the following extended table: {CUSTOMER, COUNTRY}
- The COUNTRY base table will have the following extended table: {COUNTRY}
The INVOICE extended table includes not only its own data but also data from the CUSTOMER and COUNTRY tables. This is because, starting from the INVOICE table (base table), there is an N-1 relationship with the CUSTOMER table; and starting from the CUSTOMER table, there is an N-1 relationship with the COUNTRY table. Note that if you take one record from INVOICE, you find only one record from CUSTOMER related to it (and no more than one); and if you take one record from CUSTOMER, you find only one record from COUNTRY related to it (and no more than one).
Following the same reasoning, the CUSTOMER extended table includes not only its own data but also data from the COUNTRY table. This is because, starting from the CUSTOMER table (base table) there is an N-1 relationship with the COUNTRY table. Note that if you take one record from CUSTOMER, you find only one record from COUNTRY related to it (and no more than one).
The COUNTRY extended table contains only itself. This is because, starting from it, there is no N-1 relationship either directly or indirectly.
The extended table concept simplifies access to multiple tables when working from a specific Base Table.
Base and Extended table