Computer Programming Concepts and Visual Basic David I. Schneider

334 Computer Programming Concepts and Visual Basic
III (b) Show the records from the joined table whose currency has “u” as its second letter.
SELECT * FROM Cities INNER JOIN Countries ON Cities.country =
Countries.country WHERE currency Like ‘?u*’
IV (a) Make available just the city and country fields of the table Cities.
SELECT city, country FROM Cities
IV (b) Make available just the city and currency fields of the joined table.
SELECT city,, currency FROM Cities INNER JOIN Countries ON Cities.country
= Countries.country
Note: In several of the statements, the single quote, rather than the normal double quote was
used to surround strings. This is standard practice with SQL statements.
We can think of an SQL statement as creating in essence a new “virtual” table from
existing tables. For instance, we might regard the statement
SELECT city, pop2015 FROM Cities WHERE pop2015 >= 20
as creating the “virtual” table
city pop2015
Tokyo 28.7
Sao Paulo 20.8
Bombay 27.4
Shanghai 23.4
This table is a subtable of the original table Cities, that is, it consists of what is left after certain
columns and rows are deleted.
As another example, the statement
SELECT Cities.city, Cities.Country, Country.currency FROM Cities INNER JOIN
Countries ON Cities.country = Countries.country WHERE Countries.country>’K’
creates in essence the “virtual” table
Cities.city Cities.country currency
New York USA dollar
Mexico City Mexico peso
Los Angeles USA dollar
which is a subtable of a join of the two tables Cities and Countries.
These “virtual” tables don’t really exist physically. However, for all practical purposes,
Visual Basic acts as if they did. In Visual Basic terminology, a “virtual” table is called a
recordset and SQL statements are said to create a recordset. In standard relational database
books, a “virtual” table is called a view.
SQL also can be used in code with a statement of the form
Data1.RecordSource = “ SELECT ... FROM ...”
to alter the order and kinds of records presented from a database. However, such a statement
must be followed by the statement
Data1.Refresh
to reset the information processed by the data control.
EXAMPLE 1
The following program allows the user to alter the order and kinds of information displayed from a database.
When the first command button is pressed, the cities are presented in ascending order based on their