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Chapter 5 Database Processing
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Figure 5-5
Components of a Database /GVCFCVC

Relationships Among Rows

Consider the terms on the left-hand side of Figure 5-5. You know what tables are. To understand what
is meant by relationships among rows in tables, examine Figure 5-6. It shows sample data from the three
tables Email, Student, and Office_Visit. Notice the column named Student Number in the Email table.
That column indicates the row in Student to which a row of Email is connected. In the first row of
Email, the Student Number value is 1325. This indicates that this particular email was received from
the student whose Student Number is 1325. If you examine the Student table, you will see that the row
for Andrea Baker has this value. Thus, the first row of the Email table is related to Andrea Baker.
Now consider the last row of the Office_Visit table at the bottom of the figure. The value of
Student Number in that row is 4867. This value indicates that the last row in Office_Visit belongs to
Adam Verberra.
From these examples, you can see that values in one table relate rows of that table to rows in
a second table. Several special terms are used to express these ideas. A key (also called a primary
key) is a column or group of columns that identifies a unique row in a table. Student Number is the
Figure 5-6 key of the Student table. Given a value of Student Number, you can determine one and only one row
Example of Relationships in Student. Only one student has the number 1325, for example.
Among Rows

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