XSD: difference between Element and Attribute

Question

I'm new to XSD, and I'm quite confused as to when to use attribute, and when to use element?

Why cant we specify minOccurs and maxOccurs in attribute?

Also, why is it we cannot specify use="required" in element?

Answer 1

An element is an XML element - a opening tag, some content, a closing tag - they are the building blocks of your XML document:

<test>someValue</test>

Here, "test" would be an element.

Attributes is an additional info on a tag - it's an "add-on" or an extra info on an element, but can never exist alone:

<test id="5">somevalue</test>

"id" is an attribute.

You cannot have multiple attributes of the same name on a single tag --> minOccurs/maxOccurs makes no sense. You can define required (or not) for an attribute - anything else doesn't make sense.

The elements are defined by their occurrence inside complex types - e.g. if you have a complex type with a <xs:sequence> inside - you are defining that all elements must be present and must the in this particular order:

<xs:complexType name="SomeType">
   <xs:sequence>       
      <xs:element name="Element1" type="xs:string" />
      <xs:element name="Element2" type="xs:string" />
   </xs:sequence>
</xs:complexType>

Inside an element of that type, the sub-elements "Element1" and "Element2" are required and must appear in this order - there's no need for "required" or not (like with attributes). Whether or not an element is required is defined by the use of minOccurs and maxOccurs; both are =1 by default, e.g. the element must occur, and can only occur once. By tweaking those settings, you can define an element to be optional (minOccurs=0), or allow it to show up several times (maxOccurs > 1).

I'd strongly recommend you check out the W3Schools Tutorial on XML Schema and learn some more about XML schema.

Marc