What is difference between XML Schema and DTD

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I have googled this question  but I do not understand clearly what is an XML schema and DTD  document type definition   and why the XML schema is more powerful compared to DTD    Any guidance would be highly appreciated

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DTD can have only two types of data  the CDATA and the PCDATA  But in a schema you can use all the primitive data type that you use in the programming language and you have the flexibility of defining your own custom data types   The developer building a schema can create custom data types based on the core data types and by using different operators and modifiers

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When XML first came out  we were told it would solve all our problems   XML will be user-friendly  infinitely extensible  avoid strong-typing  and not require any programming skills   I learnt about DTD s and wrote my own XML parser   15  years later  I see that most XML is not user-friendly  and not very extensible  depending on its usage    As soon as some clever clogs hooked up XML to a database I knew that data types were all but inevitable   And  you should see the XSLT  transformation file  I had to work on the other day   If that isn t programming  I don t know what is   Nowadays it s not unusual to see all kinds of problems relating to XML data or interfaces gone bad   I love XML but  it has strayed far from its original altruistic starting point     The short answer   DTD s have been deprecated in favor of XSD s because an XSD lets you define an XML structure with more precision

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XML DTD  The purpose of a DTD is to define the structure of an XML document  It defines the structure with a list of legal elements    lt  ATTLIST contact type CDATA  IMPLIED gt   lt  ELEMENT address1    PCDATA  gt   lt  ELEMENT city    PCDATA  gt   lt  ELEMENT state    PCDATA  gt   lt  ELEMENT zip    PCDATA  gt    XML Schema  XML Schema enables schema authors to specify that element quantity   s data must be numeric or  even more specifically  an integer   In the following example I used string    lt xs element name  note  gt   lt xs complexType gt     lt xs sequence gt       lt xs element name  address1  type  xs string   gt       lt xs element name  city  type  xs string   gt       lt xs element name  state  type  xs string   gt       lt xs element name  zip  type  xs string   gt     lt  xs sequence gt   lt  xs complexType gt

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Differences between an XML Schema Definition  XSD  and Document Type Definition  DTD  include   XML schemas are written in XML while DTD are derived from SGML syntax  XML schemas define datatypes for elements and attributes while DTD doesn t support datatypes  XML schemas allow support for namespaces while DTD does not  XML schemas define number and order of child elements  while DTD does not  XML schemas can be manipulated on your own with XML DOM but it is not possible in case of DTD  using XML schema user need not to learn a new language but working with DTD is difficult for a user  XML schema provides secure data communication i e sender can describe the data in a way that receiver will understand  but in case of DTD data can be misunderstood by the receiver  XML schemas are extensible while DTD is not extensible   Not all these bullet points are 100  accurate  but you get the gist  On the other hand   DTD lets you define new ENTITY values for use in your XML file  DTD lets you extend it local to an individual XML file

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Similarities   DTDs and Schemas both perform the same basic functions    First  they both declare a laundry list of elements and attributes  Second  both describe how those elements are grouped  nested or used within the XML  In other words  they declare the rules by which you are allowing someone to create an XML file within your workflow  and Third  both DTDs and schemas provide methods for restricting  or forcing  the type or format of an element  For example  within the DTD or Schema you can force a date field to be written as 01 05 06 or 1 5 2006    Differences     DTDs are better for text-intensive applications  while schemas have several advantages for data-intensive workflows   Schemas are written in XML and thusly follow the same rules  while DTDs are written in a completely different language    Examples   DTD    lt  xml version  1 0  encoding  UTF-8   gt       lt  ELEMENT employees  Efirstname  Elastname  Etitle  Ephone  Eemail  gt            lt  ELEMENT Efirstname   PCDATA  gt            lt  ELEMENT Elastname   PCDATA  gt            lt  ELEMENT Etitle   PCDATA  gt            lt  ELEMENT Ephone   PCDATA  gt            lt  ELEMENT Eemail   PCDATA  gt    XSD    lt  xml version  1 0  encoding  UTF-8   gt   lt xsd schema xmlns xsd  http   www w3 org 2001 XMLSchema  xmlns od  urn schemas-microsoft-com officedata  gt   lt xsd element name  dataroot  gt        lt xsd complexType gt             lt xsd sequence gt                  lt xsd element ref  employees  minOccurs  0  maxOccurs  unbounded   gt             lt  xsd sequence gt             lt xsd attribute name  generated  type  xsd dateTime   gt         lt  xsd complexType gt   lt  xsd element gt   lt xsd element name  employees  gt         lt xsd annotation gt              lt xsd appinfo gt                  lt od index index-name  PrimaryKey  index-key  Employeeid   primary  yes                  unique  yes  clustered  no   gt             lt od index index-name  Employeeid  index-key  Employeeid   primary  no  unique  no             clustered  no   gt        lt  xsd appinfo gt   lt  xsd annotation gt        lt xsd complexType gt             lt xsd sequence gt                  lt xsd element name  Elastname  minOccurs  0  od jetType  text                  od sqlSType  nvarchar  gt                       lt xsd simpleType gt                            lt xsd restriction base  xsd string  gt                                 lt xsd maxLength value  50   gt                            lt  xsd restriction gt                       lt  xsd simpleType gt                  lt  xsd element gt                  lt xsd element name  Etitle  minOccurs  0  od jetType  text  od sqlSType  nvarchar  gt                       lt xsd simpleType gt                            lt xsd restriction base  xsd string  gt                                 lt xsd maxLength value  50   gt                            lt  xsd restriction gt                       lt  xsd simpleType gt                  lt  xsd element gt                  lt xsd element name  Ephone  minOccurs  0  od jetType  text                  od sqlSType  nvarchar  gt                       lt xsd simpleType gt                            lt xsd restriction base  xsd string  gt                                 lt xsd maxLength value  50   gt                            lt  xsd restriction gt                       lt  xsd simpleType gt                  lt  xsd element gt                  lt xsd element name  Eemail  minOccurs  0  od jetType  text                 od sqlSType  nvarchar  gt                       lt xsd simpleType gt                            lt xsd restriction base  xsd string  gt                                 lt xsd maxLength value  50   gt                            lt  xsd restriction gt                       lt  xsd simpleType gt                  lt  xsd element gt                  lt xsd element name  Ephoto  minOccurs  0  od jetType  text                  od sqlSType  nvarchar  gt                       lt xsd simpleType gt                            lt xsd restriction base  xsd string  gt                                 lt xsd maxLength value  50   gt                            lt  xsd restriction gt                       lt  xsd simpleType gt                  lt  xsd element gt             lt  xsd sequence gt   lt  xsd complexType gt   lt  xsd element gt   lt  xsd schema gt

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DTD indicates the syntax of the XML element   XML Schemas are Microsoft s alternative to DTD for validating XML

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From the Differences Between DTDs and Schema section of the Converting a DTD into a Schema article      The critical difference between DTDs   and XML Schema is that XML Schema   utilize an XML-based syntax  whereas   DTDs have a unique syntax held over   from SGML DTDs  Although DTDs are   often criticized because of this need   to learn a new syntax  the syntax   itself is quite terse  The opposite is   true for XML Schema  which are   verbose  but also make use of tags and   XML so that authors of XML should find   the syntax of XML Schema less   intimidating       The goal of DTDs was to retain a level   of compatibility with SGML for   applications that might want to   convert SGML DTDs into XML DTDs    However  in keeping with one of the   goals of XML   terseness in XML markup   is of minimal importance   there is no   real concern with keeping the syntax   brief                  So what are some of the other differences which might be especially important when we are converting a DTD  Let s take a look       Typing      The most significant difference between DTDs and XML Schema is the capability to create and use datatypes in Schema in conjunction with element and attribute declarations  In fact  it s such an important difference that one half of the XML Schema Recommendation is devoted to datatyping and XML Schema  We cover datatypes in detail in Part III of this book   XML Schema Datatypes                   Occurrence Constraints      Another area where DTDs and Schema differ significantly is with occurrence constraints  If you recall from our previous examples in Chapter 2   Schema Structure   or your own work with DTDs   there are three symbols that you can use to limit the number of occurrences of an element       and                    Enumerations      So  let s say we had a  element  and we wanted to be able to define a size attribute for the shirt  which allowed users to choose a size  small  medium  or large  Our DTD would look like this    lt  ELEMENT item  shirt  gt   lt  ELEMENT shirt   PCDATA  gt   lt  ATTLIST shirt     size value  small   medium   large  gt                   But what if we wanted size to be an element  We can t do that with a DTD  DTDs do not provide for enumerations in an element s text content  However  because of datatypes with Schema  when we declared the enumeration in the preceding example  we actually created a simpleType called size values which we can now use with an element    lt xs element name  size  type  size value  gt

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DTD is pretty much deprecated because it is limited in its usefulness as a schema language  doesn t support namespace  and does not support data type  In addition  DTD s syntax is quite complicated  making it difficult to understand and maintain

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One difference is that in a DTD the content model of an element is completely determined by its name  independently of where it appears in the document   Assuming you want to have   a person element with a child element called name   an name itself has child elements first and last     Like this      lt person gt          lt name gt               lt first gt  lt  first gt               lt last gt  lt  last gt          lt  name gt      lt  person gt    If a city element in the same document also needs to have a child element  name  the DTD requires that this  name  element must have child elements first and last as well  Despite the fact that city name does not require first and last as children    In contrast  XML Schema allows you to declare child element types locally  you could declare the name child elements for both person and city separately  Thus giving them their proper content models in those contexts   The other major difference is support for namespaces  Since DTDs are part of the original XML specification  and inherited from SGML   they are not namespace-aware at all because XML namespaces were specified later  You can use DTDs in combination with namespaces  but it requires some contortions  like being forced to define the prefixes in the DTD and using only those prefixes  instead of being able to use arbitrary prefixes   To me  other differences are mostly superficial  Datatype support could easily be added to DTDs  and syntax is just syntax   I  for one  find the XML Schema syntax horrible and would never want to hand-maintain an XML Schema  which I wouldn t say about DTDs or RELAX NG schemas  if I need an XML Schema for some reason  I usually write a RELAX NG one and convert it with trang

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As many people have mentioned before  XML Schema utilize an XML-based syntax and DTDs have a unique syntax   DTD doesn t support datatypes  which does matter    Lets see a very simple example in which university has multiple students and each student has two elements  name  and  year   Please note  that I have uses     --    in my code just for comments     Now I will write this example both in DTD and in XSD   DTD   lt  xml version  1 0  encoding  UTF-8   gt   lt  DOCTYPE university                  -- gt  university as root element   lt  ELEMENT university  student   gt       -- gt  university has      Multiple students  lt  ELEMENT student  name year  gt         -- gt  Student has elements name and year  lt  ELEMENT name   PCDATA  gt              -- gt  name as Parsed character data  lt  ELEMENT year   PCDATA  gt              -- gt  year as Parsed character data   gt    lt university gt       lt student gt           lt name gt              John Niel               --- gt  I can also use an Integer not good          lt  name gt           lt year gt              2000                   --- gt  I can also use a string not good          lt  year gt       lt  student gt   lt  university gt    XML Schema Definition  XSD    lt  xml version  1 0  encoding  UTF-8   gt   lt xsd schema xmlns xsd  http   www w3 org 2001 XMLSchema  gt    lt xsd complexType name   uniType  gt                       -- gt  complex datatype uniType   lt xsd sequence gt     lt xsd element ref  student  maxOccurs  unbounded   gt    -- gt  has unbounded no of students   lt  xsd sequence gt   lt  xsd complexType gt    lt xsd complexType name  stuType  gt                        -- gt  complex datatype stuType   lt xsd sequence gt     lt xsd element ref  name   gt                             -- gt  has element name    lt xsd element ref  year   gt                             -- gt  has element year   lt  xsd sequence gt   lt  xsd complexType gt    lt xsd element name  university  type  uniType   gt          -- gt  university of type UniType   lt xsd element name  student  type  stuType   gt             -- gt  student of type stuType  lt xsd element name  name  type  xsd string   gt             -- gt  name of datatype string  lt xsd element name  year  type  xsd integer   gt            -- gt  year of datatype integer  lt  xsd schema gt      lt  xml version  1 0  encoding  UTF-8   gt   lt university gt       lt student gt           lt name gt              John Niel                    lt  name gt           lt year gt              2000                        -- gt  only an Integer value is allowed          lt  year gt       lt  student gt   lt  university gt

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Similarities between XSD and DTD  both specify elements  attributes  nesting  ordering   occurences   Differences between XSD and DTD  XSD also has data types   typed  pointers  namespaces  keys and more     unlike DTD    Moreover though XSD is little verbose its syntax is extension of XML  making it convenient to learn fast

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DTD predates XML and is therefore not valid XML itself  That s probably the biggest reason for XSD s invention

[xml] What is difference between XML Schema and DTD?

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