One-One, One-Many and Many-Many Relations in XML Schema - Implementing a Many to Many relation in XML Schema: the schema
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Until now, we have gone through only one to one and one to many relations. Now, it is time to look at implementing a many to many relation. Many to many relations are a bit complicated when compared with the above two.
For the sake of this example, I am taking the same concept of the “pubs” database (demonstration database) existing in Microsoft SQL Server. Concisely, it has three tables: “Titles,” “Authors” and “TitleAuthors”. It also has relations in between three of them. I would like to have the same concept implemented in my schema (instead of in the database).
Let us consider the following schema.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" attributeFormDefault=
"unqualified">
<xs:element name="pubs">
<xs:complexType>
<xs:sequence>
<xs:element name="Authors"
type="TypeAuthors" maxOccurs="unbounded" />
<xs:element name="Titles"
type="TypeTitles" maxOccurs="unbounded" />
<xs:element name="TitleAuthors"
type="TypeTitleAuthor" maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
.
.
.
<xs:complexType name="TypeTitles">
<xs:sequence>
<xs:element name="Title_ID" type="xs:string" />
<xs:element name="Title" type="xs:string" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="TypeAuthors">
<xs:sequence>
<xs:element name="au_id" type="xs:string" />
<xs:element name="au_lname" type="xs:string" />
<xs:element name="au_fname" type="xs:string" />
</xs:sequence>
</xs:complexType>
<xs:complexType name="TypeTitleAuthor">
<xs:sequence>
<xs:element name="au_id" type="xs:string" />
<xs:element name="Title_ID" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:schema>
Finally, here are the primary keys and foreign keys -- the leftovers of the schema presented above. The following is the fragment schema you need to add to the schema defined above (at the appropriate place only).
<xs:key name="PK_Authors">
<xs:selector xpath=".//Authors" />
<xs:field xpath="au_id" />
</xs:key>
<xs:key name="PK_Titles">
<xs:selector xpath=".//Titles" />
<xs:field xpath="Title_ID" />
</xs:key>
<xs:key name="PK_TitleAuthors">
<xs:selector xpath=".//TitleAuthors" />
<xs:field xpath="au_id" />
<xs:field xpath="Title_ID" />
</xs:key>
<xs:keyref name="FK_Authors_TitleAuthors"
refer="PK_Authors">
<xs:selector xpath=".//TitleAuthors" />
<xs:field xpath="au_id" />
</xs:keyref>
<xs:keyref name="FK_Titles_TitleAuthors"
refer="PK_Titles">
<xs:selector xpath=".//TitleAuthors" />
<xs:field xpath="Title_ID" />
</xs:keyref>
You can observe carefully that I have three primary keys and two foreign keys to represent a “many-many” relation (of course, you can have even more).
Any comments, suggestions, feedback, bugs, errors are highly appreciated at jag_chat@yahoo.com
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