Introduction
In the last few installments of the XML Workshop, I had been trying to give a detailed explanation of some of the building blocks of XSD. There are a few more points that we still need to discuss before closing down the XSD section of the XML Workshop.
XML is getting widely accepted as the format for data exchange. Wide adoption of architectures like SOA (Service Oriented Architecture) contributed to the growth of applications and services that talk to each other and exchange data in XML format. When applications exchange data, there will be strict rules that define the structure as well as the quality of data. If the data does not adhere to the rules, it may not be useful at all.
XSD helps you to define those rules which will validate the structure and quality of the data that your application sends or receives. Complex validation rules can be broken down to simple XSD definitions and can be stored to an XML SCHEMA COLLECTION. You could then ask SQL Server 2005 to validate the data based on the definitions in the XML SCHEMA COLLECTION.
In the previous few sessions, we have seen how to define rules which validate the structure of an XML document. We also learned how to define rules which validate the quality of data. Before we move out of XSD, I would like to present a few moreXSD elements and attributes which define advanced characteristics of an XML document.
In the previous sessions, we have seen XML documents, composed of XML elements and attributes. We learned that if the element is a Simple Type it contains a value. If the element is a ComplexType then it will contain other elements and/or attributes. All the elements we have seen so far hold either a value or other child elements, but not both. In this session, we are going to have a look at Mixed types. Mixed Type is an element which will contain a value as well as other child elements.
Mixed Types
Mixed types are elements which contain value as well as other child elements. Mixed types usually contain long text data like the body of a letter, e-mail message or similar data where the data itself contains other specific pieces of information which needs to be extracted separately. The following example shows a case where a Mixed Type may be used.
SQLServerCentral sends an e-mail to the author when an article is scheduled for publication. Let us design a basic XMLstructure that holds the notification details. Here is a simple schema that holds the basic information. [schema1.xml]
1 <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
2 <xs:element name="ArticleNotification">
3 <xs:complexType>
4 <xs:sequence>
5 <xs:element name="Body" type="xs:string"/>
6 <xs:element name="Title" type="xs:string" />
7 <xs:element name="Url" type="xs:string" />
8 <xs:element name="ScheduledDate" type="xs:date"/>
9 xs:sequence>
10 xs:complexType>
11 xs:element>
12 xs:schema>
The above SCHEMA defines the following XML structure.
1 <ArticleNotification>
2 <Body>The following article is scheduled for publication.Body>
3 <Title>XML Workshop VII - Validating values with SCHEMATitle>
4 <Url>http://www.sqlservercentral.com/3120.aspUrl>
5 <ScheduledDate>2007-01-01ZScheduledDate>
6 ArticleNotification>
Mixed Mode gives us the freedom to alter the XML structure to be more descriptively. For example, the following XML structure is preferable over the previous one in this specific case..
1 <ArticleNotification>
2 Hi<Name>JacobName>,
3 Please note that your article
4 <Title>XML Workshop VII - Validating values with SCHEMATitle>,
5 posted at <Url>http://www.sqlservercentral.com/3120.aspUrl> is
6 scheduled for publication
7 on <ScheduledDate>2007-01-01ScheduledDate>.
8 ArticleNotification>
If you would like to be a little more concise, you could go for a better structure as given below.
1 <ArticleNotification Url="http://www.sqlservercentral.com/3120.asp">
2 Hi<Name>JacobName>,
3 Please note that your article
4 <Title>XML Workshop VII - Validating values with SCHEMATitle>
5 is scheduled for publication
6 on <ScheduledDate>2007-01-01ScheduledDate>.
7 ArticleNotification>
The XML structure I presented above is different from the structures that we have seen so far. In the above example, we have an element which has a value, an attribute and has other child elements. So, how do we define such elements in XSD? The answer is "by using Mixed Types". Let us write the XSD for the above XML structure. [Download schema2.xml]
1 <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
2 <xs:element name="ArticleNotification">
3 <xs:complexType mixed="true">
4 <xs:sequence>
5 <xs:element name="Name" type="xs:string" />
6 <xs:element name="Title" type="xs:string"/>
7 <xs:element name="ScheduledDate" type="xs:date"/>
8 xs:sequence>
9 <xs:attribute name="Url" type ="xs:string" />
10 xs:complexType>
11 xs:element>
12 xs:schema>
Let us create the SCHEMA collection. [Download sql1.sql
1 CREATE XML SCHEMA COLLECTION ArticleNotificationSchema AS '
2
3
4
5
6
7
8
9
10
11
12
13
14 '
15 GO
16
17 /*
18 Let us test the schema by assigning a value with the
19 desired structure to an XML variable which is bound to the
20 above schema.
21 */
22
23 DECLARE @art AS XML(ArticleNotificationSchema)
24 SET @art = '
25
26 Hi
27 Please note that your article
28
29 is scheduled for publication
30 on
31
32 '
Note that I have added "Z" at the end of the date value. If we don't do that, SQL Server will generate an error message. SQL Server 2005's implementation of "xs:date" data type takes Time Zone information along with a date value. SQL Server 2005 will not accept a date value without the Time Zone information. The following XML is equally valid. [Download sql2.sql]
1 DECLARE @art AS XML(ArticleNotificationSchema)
2 SET @art = '
3
4 Hi
5 Please note that your article
6
7 is scheduled for publication
8 on
9
10 '
Note the value "+05:30" which indicates Indian Standard Time, which is GMT+05:30.
Let us try to read information from the XML variable that we just created. Reading some of the information from the XML variable might seem little tricky. [Download sql3.sql]
1 DECLARE @art AS XML(ArticleNotificationSchema)
2 SET @art = '
3
4 Hi
5 Please note that your article
6
7 is scheduled for publication
8 on
9
10 '
11
12 /*
13 Let us make sure that the value is stored correctly.
14 Try to retrieve the "Url" attribute from the XML.
15 */
16 SELECT
17 x.a.value('(@Url)[1]', 'varchar(50)') AS Url
18 FROM @art.nodes('/ArticleNotification') x(a)
19
20 /*
21 OUTPUT:
22
23 Url
24 ----------------------------------------
25 http://www.sqlservercentral.com/3120.asp
26
27 (1 row(s) affected)
28 */
29
30 /*
31 Reading "name" and "title" is pretty simple.
32 */
33
34 SELECT
35 x.a.value('(Name)[1]', 'varchar(10)') AS [Name],
36 x.a.value('(Title)[1]','varchar(50)') AS Title,
37 x.a.value('(ScheduledDate)[1]','datetime') AS ScheduledDate
38 FROM @art.nodes('/ArticleNotification') x(a)
39
40 /*
41 OUTPUT:
42
43 Name Title ScheduledDate
44 ------ ------------------------------------------------ -------------------
45 Jacob XML Workshop VII - Validating values with SCHEMA 2007-01-01 00:00:00
46
47 (1 row(s) affected)
48
49 */
Good so far. Now how do we read the content of the ArticleNotification element? How do we retrieve the text from a mixed element? Here is the TSQL query which retrieves the whole text of the mixed element. [Download sql4.sql]
1 DECLARE @art AS XML(ArticleNotificationSchema)
2 SET @art = '
3
4 Hi
5 Please note that your article
6
7 is scheduled for publication
8 on
9
10 '
11
12 /*
13 Read the entire text from the "ArticleNotification" element.
14 */
15
16 SELECT
17 x.a.value('(data(/))[1]', 'varchar(max)') AS Summary
18 FROM @art.nodes('/ArticleNotification') x(a)
19
20 /*
21 OUTPUT:
22
23 Summary
24 ---------------------------------------------------
25
26 Hi Jacob,
27 Please note that your article
28 XML Workshop VII - Validating values with SCHEMA
29 is scheduled for publication
30 on 2007-01-01Z.
31
32
33 (1 row(s) affected)
34
35 */
We have not looked into XQUERY yet. The syntax and keywords used in the above queries may be little confusing to some of you. We will soon do a good review of XQUERY in the next few sessions.
Conclusions
In the last few sessions, we had been trying to understand XSD implementation of SQL Server 2005 better. There are a few more XSD points to discuss and then we will move to XQUERY