Computer Programming Concepts and Visual Basic David I. Schneider

330 Computer Programming Concepts and Visual Basic
to Example 1, the program will find the database when it is located in the same
folder as the program. The assignment made in the Form_Load event procedure
will override any setting made in the Properties window. (The setting in the
Properties window even can be left blank.) The programs for Section 11 on the
CD accompanying this textbook contain the above Form_Load event procedure.
2. You will most likely alter the file MEGACTY1.MDB while experimenting with
the data control or running the program in Example 1. You can always obtain a
fresh copy of MEGACTY1.MDB by recopying it from the CD.
3. You can prevent the user from altering the data in a table by setting the Read-
Only property of its data control to True.
4. The following controls can be bound to a data control: text box, check box,
image, label, picture box, list box, combo box, data bound list box, data bound
combo box, and FlexGrid.
5. A form can contain more than one data control.
6. Some entries in a table can be empty. For instance, in the Cities table, if the
2015 projected value is not known for a particular city, it can be omitted.
7. Do not use a method such as Move, Delete, or AddNew inside the Validate event
procedure. Otherwise, an infinite loop will occur.
8. Field names can be up to 64 characters in length and can consist of letters, numbers,
and spaces. If spaces are used in a name, then the name must be enclosed
in brackets when used in Visual Basic.
9. Both tables in the database MEGACTY1.MDB have fields called country. If
there is ever any question about which is being referred to, we can distinguish
them by using the two (full) names Cities.country and Countries. country.
10. In the MEGACTY1.MDB database, the values in the field city are all of data
type String and the values in the field pop1995 are all of data type Single. We
say that field city has type String (also known as Text) and the field pop1995
has type Single. Two other common data types are Date/Time and Boolean (also
known as Yes/No).
11. When a field is first created, a type must be specified. When that type is String
(or Text), a maximum length must also be specified. In the MEGACTY1.MDB
database, the fields city and country have maximum length 20 and the field currency
has maximum length 10.
12. The database MEGACTY1.MDB was created with Visual Data Manager, which
has the same format as Access. When the database to be used has been created
with other software, such as FoxPro 3.0 or dBase 5.0, then the walkthrough
requires an additional step. Namely, between Steps 3 and 4, the Connect property
of the data control has to be set to the name of the software product. (This
step was not necessary in our case because Access is the default software.)
Note: Access database file names end with .MDB, which is an abbreviation for
Microsoft Data Base. Btrieve, FoxPro, dBase, and Paradox database file names
end with .DAT, .DBF, .DBF, and .DB, respectively.
11.2 RELATIONAL DATABASES AND SQL
■ PRIMARY AND FOREIGN KEYS
A well-designed table should have a field (or set of fields) that can be used to uniquely identify
each record. Such a field (or set of fields) is called a primary key. For instance, in the
Countries table of Section 11.1, the country field is a primary key. In the Cities table, because
we are only considering very large cities (of over 1 million population), the city field is a pri-