[xml] What is the purpose of XSD files?

Since we can query on the XML file from C# (.NET), why do we need an XSD file? I know it is metadata file of particular XML file. We can specify the relationships in XSD, but what is its functioning then?

XML

<?xml version="1.0" encoding="utf-8" ?>
<Root>
  <Customers>
    <Customer CustomerID="GREAL">
      <CompanyName>Great Lakes Food Market</CompanyName>
      <ContactName>Howard Snyder</ContactName>
      <ContactTitle>Marketing Manager</ContactTitle>
      <Phone>(503) 555-7555</Phone>
      <FullAddress>
        <Address>2732 Baker Blvd.</Address>
        <City>Eugene</City>
        <Region>OR</Region>
        <PostalCode>97403</PostalCode>
        <Country>USA</Country>
      </FullAddress>
    </Customer>
  </Customers>
  <Orders>
    <Order>
      <CustomerID>GREAL</CustomerID>
      <EmployeeID>6</EmployeeID>
      <OrderDate>1997-05-06T00:00:00</OrderDate>
      <RequiredDate>1997-05-20T00:00:00</RequiredDate>
      <ShipInfo ShippedDate="1997-05-09T00:00:00">
        <ShipVia>2</ShipVia>
        <Freight>3.35</Freight>
        <ShipName>Great Lakes Food Market</ShipName>
        <ShipAddress>2732 Baker Blvd.</ShipAddress>
        <ShipCity>Eugene</ShipCity>
        <ShipRegion>OR</ShipRegion>
        <ShipPostalCode>97403</ShipPostalCode>
        <ShipCountry>USA</ShipCountry>
      </ShipInfo>
    </Order>
    <Order>
      <CustomerID>GREAL</CustomerID>
      <EmployeeID>8</EmployeeID>
      <OrderDate>1997-07-04T00:00:00</OrderDate>
      <RequiredDate>1997-08-01T00:00:00</RequiredDate>
      <ShipInfo ShippedDate="1997-07-14T00:00:00">
        <ShipVia>2</ShipVia>
        <Freight>4.42</Freight>
        <ShipName>Great Lakes Food Market</ShipName>
        <ShipAddress>2732 Baker Blvd.</ShipAddress>
        <ShipCity>Eugene</ShipCity>
        <ShipRegion>OR</ShipRegion>
        <ShipPostalCode>97403</ShipPostalCode>
        <ShipCountry>USA</ShipCountry>
      </ShipInfo>
    </Order>
  </Orders>
</Root>

I want to get data from the Order elements according to a provided CustomerID.

Also: What is the purpose of giving the relationships in XSD?

XSDs constrain the vocabulary and structure of XML documents.

• Without an XSD, an XML document need only follow the rules for being well-formed as given in the W3C XML Recommendation.
• With an XSD, an XML document must adhere to additional constraints placed upon the names and values of its elements and attributes in order to be considered valid against the XSD per the W3C XML Schema Recommendation.

XML is all about agreement, and XSDs provide the means for structuring and communicating the agreement beyond the basic definition of XML itself.

An XSD file is an XML Schema Definition and it is used to provide a standard method of checking that a given XML document conforms to what you expect.

An XSD is a formal contract that specifies how an XML document can be formed. It is often used to validate an XML document, or to generate code from.

The xsd file is the schema of the xml file - it defines which elements may occur and their restrictions (like amount, order, boundaries, relationships,...)

Without XML Schema (XSD file) an XML file is a relatively free set of elements and attributes. The XSD file defines which elements and attributes are permitted and in which order.

In general XML is a metalanguage. XSD files define specific languages within that metalanguage. For example, if your XSD file contains the definition of XHTML 1.0, then your XML file is required to fit XHTML 1.0 rather than some other format.

Before understanding the XSD(XML Schema Definition) let me explain;

What is schema?

for example; emailID: peter#gmail

You can identify the above emailID is not valid because there is no @, .com or .net or .org.

We know the email schema it looks like [email protected].

Conclusion: Schema does not validate the data, It does the validation of structure.

XSD is actually one of the implementation of XML Schema. others we have relaxng

We use XSD to validate XML data.

Also questions is: What is the purpose of giving the relationships in xsd.

Suppose you want to generate some XML for an external party's tool, or similar - how would you know what structure it is allowed to follow to be used correctly for their tool? you write to a schema. Likewise if you want other people to use your tool, you would write a schema for them to follow. It may also be useful for validating your own XML.

You mention C# in your question so it may help to think of as XSD as serving a similar role to a C# interface.

It defines what the XML should 'look like' in a similar way that an interface defines what a class should implement.

An .xsd file is called an XML schema. Via an XML schema, we may require a certain structure in a given XML - which elements in which order, how many times, with which attributes, how they are nested, etc. If we have a schema for our XML input, we can verify that it contains the data we need it to contain, and nothing else, with a few lines invoking a schema validator.