In relational terminology, selection is defined as
taking the horizontal subset of rows of a single table that satisfies
a particular condition. This kind of SELECT statement returns some
of the rows and all the columns in a table. Selection is implemented
through the WHERE clause of a SELECT statement, as the following figure
shows. Figure 1: Query
SELECT * FROM customer WHERE state = 'NJ';
The result contains the same number of columns as the customer table,
but only a subset of its rows. In this example, DB-Access displays
the data from each column on a separate line. Figure 2: Query result
customer_num 119
fname Bob
lname Shorter
company The Triathletes Club
address1 2405 Kings Highway
address2
city Cherry Hill
state NJ
zipcode 08002
phone 609-663-6079
customer_num 122
fname Cathy
lname O‘Brian
company The Sporting Life
address1 543d Nassau
address2
city Princeton
state NJ
zipcode 08540
phone 609-342-0054
In relational terminology, projection is defined as
taking a vertical subset from the columns of a single table that retains
the unique rows. This kind of SELECT statement returns some of the
columns and all the rows in a table.
Projection is implemented through the projection list in
the Projection clause of a SELECT statement, as the following figure
shows. Figure 3: Query
SELECT city, state, zipcode FROM customer;
The result contains the same number of rows
as the customer table, but it projects only a
subset of the columns in the table. Because only a small amount of
data is selected from each row, DB-Access is
able to display all of the data from the row on one line. Figure 4: Query result
city state zipcode
Sunnyvale CA 94086
San Francisco CA 94117
Palo Alto CA 94303
Redwood City CA 94026
Los Altos CA 94022
Mountain View CA 94063
Palo Alto CA 94304
Redwood City CA 94063
Sunnyvale CA 94086
Redwood City CA 94062
Sunnyvale CA 94085
;
Oakland CA 94609
Cherry Hill NJ 08002
Phoenix AZ 85016
Wilmington DE 19898
Princeton NJ 08540
Jacksonville FL 32256
Bartlesville OK 74006
The most common kind of SELECT statement uses both selection and
projection. A query of this kind returns some of the rows and some
of the columns in a table, as the following figure shows. Figure 5: Query
SELECT UNIQUE city, state, zipcode
FROM customer
WHERE state = 'NJ';
Query result contains a subset
of the rows and a subset of the columns in the customer table. Figure 6: Query result
city state zipcode
Cherry Hill NJ 08002
Princeton NJ 08540