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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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