Database Reference
In-Depth Information
Figure 8.6. Does the primary key exclusively identify the value of each field in this
table?
First,youloadthetablewithsampledata.Youthenselectarecordfortestpurposes—we'll
usethethirdrecordforthisexample—andnotethevalueoftheprimarykey(13002).Now,
pose the following question for each field value in the record.
Does this primary key value
exclusively
identify the current value of . . .
I
NVOICE
D
ATE
? Yes, it does. This invoice number will always identify the spe-
cific date that the invoice was created.
C
UST
F
IRST
N
AME
? Yes, it does. This invoice number will always identify the spe-
cific first name of the particular customer who made this pur-
chase.
C
UST
L
AST
N
AME
? Yes, it does. This invoice number will always identify the specif-
ic last name of the particular customer who made this pur-
chase.
E
MP
F
IRST
N
AME
? Yes, it does. This invoice number will always identify the specif-
ic first name of the particular employee who served the cus-
tomer for this sale.
E
MP
L
AST
N
AME
? Yes, it does. This invoice number will always identify the spe-
cific last name of the particular employee who served the cus-
tomer for this sale.
E
MP
H
OME
P
HONE
?
No,
it doesn't! The invoice number
indirectly
identifies the em-
ployee's home phone number via the employee's name. In fact,
it is the
current value
of both E
MP
F
IRST
N
AME
and E
MP
L
AST
N
AME
that exclusively identifies the value of E
MP
H
OME
P
HONE
—change the employee's name and you
must
change the
phone number as well. You should now remove E
MP
H
OME
