Using this rule, we'll create attributes on the elements we have
already defined to hold the column values from our database. In a
DTD, these attributes should appear in the !ATTLIST declaration of
the element corresponding to the table in which the column
appears.
If a column is a foreign key joining to another table, don't
include it in this rule - we'll handle foreign key columns later in
the process, when we model the relationships between the elements we
have created.
Declare each attribute created this way as having the type CDATA.
If the column is defined in your database as not allowing NULL
values, then make the corresponding attribute #REQUIRED; otherwise,
make the corresponding attribute #IMPLIED.
We have four choices here. #FIXED means the DTD provides the
value. #REQUIRED means it must appear in the document. #IMPLIED means
that it may or may not appear in the document. Finally, a value with
these means that the processor must substitute that value for the
attribute
if it is not provided in the document. #IMPLIED is the only way to
legitimately leave off an attribute value.
If we choose to store table column values as the content of
elements, rather than attributes, we can take the same approach -
create an element for each data point, and add it to the content list
of the element for the table in which the column appears. Use no
suffix if the column does not allow nulls; or the optional suffix (?)
if the column allows nulls. Be aware that if we take this approach,
we'll need to be on the look out for possible name collisions between
columns in different tables with the same name. This is not an issue
when using attributes.
To summarise:
|
Does the column allow NULLS?
|
Elements
|
Attributes
|
|
Allows NULLS
|
Use the ? suffix
|
Declare as #IMPLIED
|
|
Doesn't allow NULLS
|
Use no suffix
|
Declare as #REQUIRED
|
For our example, remember that we want to keep all the nonforeign
key columns, with the exception of the system-generated primary
keys:
<!ELEMENT SalesData EMPTY>
<!ATTLIST SalesData
Status (NewVersion | UpdatedVersion | CourtesyCopy) #REQUIRED>
<!ELEMENT Invoice EMPTY>
<!ATTLIST Invoice
InvoiceNumber CDATA #REQUIRED
TrackingNumber CDATA #REQUIRED
OrderDate CDATA #REQUIRED
ShipDate CDATA #REQUIRED>
<!ELEMENT Customer EMPTY>
<!ATTLIST Customer
Name CDATA #REQUIRED
Address CDATA #REQUIRED
City CDATA #REQUIRED
State CDATA #REQUIRED
PostalCode CDATA #REQUIRED>
<!ELEMENT Part EMPTY>
<!ATTLIST Part
PartNumber CDATA #REQUIRED
Name CDATA #REQUIRED
Color CDATA #REQUIRED
Size CDATA #REQUIRED>
<!ELEMENT MonthlyTotal EMPTY>
<!ATTLIST MonthlyTotal
Month CDATA #REQUIRED
Year CDATA #REQUIRED
VolumeShipped CDATA #REQUIRED
PriceShipped CDATA #REQUIRED>
<!ELEMENT MonthlyCustomerTotal EMPTY>
<!ATTLIST MonthlyCustomerTotal
VolumeShipped CDATA #REQUIRED
PriceShipped CDATA #REQUIRED>
<!ELEMENT MonthlyPartTotal EMPTY>
<!ATTLIST MonthlyPartTotal
VolumeShipped CDATA #REQUIRED
PriceShipped CDATA #REQUIRED>
<!ELEMENT LineItem EMPTY>
<!ATTLIST LineItem
Quantity CDATA #REQUIRED
Price CDATA #REQUIRED>
Note that we left off Month and Year on the
<MonthlyPartTotal> and <MonthlySummaryTotal> structures,
since these will be dictated by the <MonthlyTotal> element
associated with these elements.
Rule 4: Modeling Nonforeign Key Columns.
Create an attribute for each column we have chosen to include in
our XML document (except foreign key columns). These attributes
should appear in the !ATTLIST declaration of the element
corresponding to the table in which they appear. Declare each of
these attributes as CDATA, and declare it as #IMPLIED or #REQUIRED
depending on whether the original column allowed nulls or not.
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