Computer Programming Concepts and Visual Basic David I. Schneider

mary key. Databases of student enrollments in a college usually use a field of social security
numbers as the primary key. Names would not be a good choice because there could easily be
two students having the same name.
When a database is created, a field can be specified as a primary key. If so, Visual Basic
will insist that every record have an entry in the primary key field and that the same entry
does not appear in two different records. If the user tries to enter a record with no data in the
primary key, the error message “Index or primary key can’t contain a null record.” will be
generated. If the user tries to enter a record with the same primary key data as another record,
the error message “Duplicate value in index, primary key, or relationship. Changes were
unsuccessful.” will be displayed.
When a database contains two or more tables, the tables are usually related. For instance,
the two tables Cities and Countries are related by their country field. Let’s refer to these two
fields as Cities.country and Countries.country. Notice that every entry in Cities.country appears
uniquely in Countries.country and Countries.country is a primary key. We say that Cities.country
is a foreign key of Countries.country. Foreign keys can be specified when a table is first created.
If so, Visual Basic will insist on the Rule of Referential Integrity, namely, that each
value in the foreign key must also appear in the primary key of the other table.
The CD accompanying this book contains a database named MEGACTY2.MDB. It has
the same information as MEGACTY1.MDB except that Cities.city and Countries.country
have been specified as primary keys for their respective tables, and Cities.country has been
specified as a foreign key of Countries.country. If the user tries to add a city to the Cities
table whose country does not appear in the Countries table, then the error message “Can’t
add or change record. Referential integrity rules require a related record in table ‘Countries’.”
will be displayed. The message will also be generated if the user tries to delete a country
from the Countries.country field that appears in the Cities.country field. Due to the
interdependence of the two tables in MEGACTY2.MDB, this database is called a relational
database.
A foreign key allows Visual Basic to link (or join) together two tables from a relational
database in a meaningful way. For instance, when the two tables Cities and Countries from
MEGACTY2.MDB are joined based on the foreign key Cities.country, the result is Table 11.4.
The record for each city is expanded to show its country’s population and its currency. This
joined table is very handy if, say, we wanted to click on navigation arrows and display a city’s
name and currency. We only have to create the original two tables; Visual Basic creates the
joined table as needed. The request for a joined table is made in a language called SQL.
Table 11.4
A Join of Two Tables
Cities. Cities. Countries. Country.
city country pop1995 pop2015 country pop1995 currency
Tokyo Japan 26.8 28.7 Japan 125.0 yen
Sao Paulo Brazil 16.4 20.8 Brazil 155.8 real
New York USA 16.3 17.6 USA 263.4 dollar
Mexico City Mexico 15.6 18.8 Mexico 85.6 peso
Bombay India 15.1 27.4 India 846.3 rupee
Shanghai China 15.1 23.4 China 1185.2 yuan
Los Angeles USA 12.4 14.3 USA 263.4 dollar
Beijing China 12.4 19.4 China 1185.2 yuan
Calcutta India 11.7 17.6 India 846.3 rupee
Tianjin China 10.7 17.0 China 1185.2 yuan
■ SQL
Relational Databases and SQL 331
Structured Query Language (SQL) was developed in the early 1970s at IBM for use with
relational databases. The language was standardized in 1986 by ANSI (American National