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In this video we'll be learning about XML schema.

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Like document type descriptors, XML

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schema allows us a way

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to give content specific specifications for our XML data.

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As you may remember, we send

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to a validating XML parser or

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XML document as well as a description.

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We talked about DTDs in the last video.

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We'll talk about XSDs in this one.

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The validating XML parser will

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check that the document is well

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formed, and it will also check that it matches it's specification.

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If it does, XML comes out.

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If it doesn't we get

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an error that the document is not valid.

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XML schema is an extensive language, very powerful.

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Like document type descriptors we

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can specify the elements we

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want in our XML data,

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the attributes, the nesting of

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the elements, how elements need

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to be ordered, and and number of occurrences of elements.

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In addition we can

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specify data types we can

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specify keys, the pointers

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that we can specify are now

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typed like in DTDs and much, much more.

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Now, one difference between XML

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schema and DTDs is that

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the specification locations in XML schemas

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called XSD's are actually written in the xml language itself.

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That can be useful for example

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if we have a browser that nicely renders the XML.

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The languages I said is vast.

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In this video, we're going

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to show one sort of quote easy example.

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But that example will give

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very much the flavor of XML schema.

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And we'll try to highlight the

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differences between XML schema and using document type descriptors.

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Ok, here were are with our XML document on the left.

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On the right we have our

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XML schema descriptor or XSD

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and we have a

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little command line that we're gonna use for our validation command.

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Now let me just say up front

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that we're not going to be

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going through the XSD line

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by line in detail the way we did with DTDs.

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As you can see it's

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rather long and that

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would take us far too long and be rather boring.

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So what I highly suggest is

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that you download the file for

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the XSD so you can

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look at it yourself and look

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at the entire file as well

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as the XML and give it a try with validating.

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What I'm gonna do in this

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demo primarily is focus

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on those aspects of

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the XSD that are different,

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are more powerful than we had in document type descriptors.

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First, let's take a look at the data itself.

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So we have our bookstore data as usual with two books and three authors.

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Its slightly restructured from any of the versions we've used before.

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It looks closest to the

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last one we used because the

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books and authors are separate

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and the authors are actually exactly the same.

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The have an identifier and a

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first name - last name sub element.

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But the primary difference is in

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the books, instead of using

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ID refs attributes to refer

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from books to authors, we still,

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we now back our back having

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an author's sub-element with the

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two authors underneath and then

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those authors themselves have what

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are effectively the pointers to the identifiers for the authors.

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And we'll see how that's

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going to mesh with the XML

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schema descriptor that we're using for this file.

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So, the other thing I want

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to mention is that right now

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we have the XML schema

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descriptor in one file and the XML in another.

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You might remember for the DTD,

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we simply placed the DTDs

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specification at the top

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of the file with the XML.

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For DTDs you can do it either way in the same file or in a separate file.

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For XSDs, we always put those in a separate file.

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Also notice that the XSD

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itself is in XML.

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It is using special tags.

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These are tags that are part

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of the XSD language, but

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we are still expressing it in XML.

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So we have two XML files, the data file and the schema file.

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To validate the data file

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against the schema file, we

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can use again the XML link feature.

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We specify the schema file,

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the data file and when

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we execute the command

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we can see that the file validates correctly.

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So I'm now going

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to highlight four features of

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XML schema that aren't present in DTD's.

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One of them is typed values.

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One of them is key declarations.

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Similar to IDs but a little bit more powerful.

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One is references which are again

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similar to pointers But a little

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more powerful and finally a currents constraints.

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So let's start with tights.

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In our data we see

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that the price attribute is

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denoted with a string and

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when we had DTDs, all attribute

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values were in fact stringed.

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In excess fees we can

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say that we want to check

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that the values which are

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still look like strings actually confirm to specific types.

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For example we can say that the price must be in integer.

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Again I'm not going to

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be labor the syntactic details but rather

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I'm just going to highlight the

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places in the XSD where

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we're declaring things of interest.

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So specifically here's where we

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declare the attribute price and

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we say that the type of price must be an integer.

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So our document validated correctly, what

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if we change this one hundred to be foo instead.

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Of course with a DTD this

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would be fine because all attributes are treated as strings.

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But if we try to validate

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now we see an error,

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specifically foo is not a value of the correct type.

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So let's change that foo back

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to a hundred so that we validate correctly.

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Next, let's talk about keys.

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In DTD's, we were able to specify ID's.

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ID's were globally unique

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values that could be

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used to identify specific elements.

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For example, when we wanted

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to point to those elements using ID refs .

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Keys are a little

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bit more powerful or more specific I should say.

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If you think about the relational model

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a key in the relational model

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is an attribute or set of

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attributes that must be

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unique for each tuple in a table.

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So, we don't have tables or

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tuples right now but, we

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do have elements and we often have repeated elements.

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So similarly we can specify

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that a particular attribute or component

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must be unique within every element of the same type.

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And we have two keys

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in our specification, one key

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which we can see here for books and one for authors.

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Specifically we say for books

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that the ISBN attribute must be a key.

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And we say for authors

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that the ident attribute must be a key.

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So let's go over to

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our data and let's start by looking at the authors.

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So if we change, for

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example, U to HG

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then we should get a key

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violation because we'll have two

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authors that have the same ident attribute.

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Let's try to validate.

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In fact, we do correctly get

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a key validation we also get

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a couple of other errors and

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those have to do with the fact

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that we are using these items as

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the destination of what are affect doubly pointers or references.

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So let's change that back

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to JU, make sure everything

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now validates fine, and it does.

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Now lets make another change.

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So we have the ident

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key here and we have

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the ISBN number, being the

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number for books, what if changed

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the ISBN number to one

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of the values we used as a key for the author, say 2HG.

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When we did something similar with

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DTDs we got an error

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because in DTDs, IDs have be globally unique.

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Here we should not get an error.

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HG should be a perfectly

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reasonable key for books because

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we don't have another value that's the same.

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And in fact it does validate.

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Now let's undo that change.

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Next, let's talk about references.

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So, references allow us to

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have what are Possibly typed

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pointers, using the dtd.

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So, they are called key

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refs, and here we

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have an example - let me just change this to the middle of the document.

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So, one of the reference types

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that we've defined in our DTD

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is a pointer to authors

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that we're using in our books.

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Specifically, we want to specify that this

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attribute here, the auth ident,

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has a value that is

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a key for the author elements.

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And we want to make sure it's

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author elements that its pointing

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to and not other types of elements.

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Now the syntax for doing

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this in XML schema is rather detailed.

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Its alright here and

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just to give you a flavor,

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this middle selector here is

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actually using the XPath language

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which we'll be using, which we'll

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be learning later but what it

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says is that when we navigate

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in the document down to one of these auth elements.

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Within that auth element, the

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auth ident attribute is

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a reference to what we

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have already defined as author keys.

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We've done something similar with books.

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We have our book  /remark/bookref

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that brings us down to this element here.

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And there we specified that the

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book attribute must be

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a reference to a book key,

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and the book key was earlier

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defined to be the ISBN number.

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Again, I know this is all

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complicated, and the syntax is

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very clunky, so I urge

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you to download the specification and spend time looking at it on your own.

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Now let's make a couple of

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changes to our document to

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demonstrate how the checking of these typed pointers works.

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For example lets change

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our first reference here to food.

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Let's validate the document and

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we should get an error and indeed

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we do, the author key rep is incorrect.

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Now lets change that FU to JW,

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so originally it was JU

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But now we're going to have two

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authors, both of whom refer to JW.

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Now this should not be a problem.

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It's simply two pointers to

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the same author, and we did

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not prohibit that in our

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XMLs schema specification and indeed our document validates.

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We'll change that one back.

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And the last, as a last

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change, we'll change our book

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reference here to refer

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to JW.

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This should not validate because this

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time, unlike with DTDs, we're,

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we've actually specified typed pointers.

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In other words, we've specified that

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this pointer or this

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reference must be to

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a book element and not to an author element.

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So we'll validate and indeed it fails.

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I've undone that change and

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now let's move to the last

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feature that we're gonna look at

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in this demonstration which is a currents constraint.

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So in, let me just

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bring up the first instance of

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it, in XML schema,

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we can specify how many

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times an element type is allowed to occur.

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Specifically we can specify the

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minimum number of occurrences and the maximum number of occurrences.

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As a default if

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we don't specify for an

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element the minOccurs or maxiOccurs the default for both of them is one.

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So here for books we've

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said that we can have

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zero Books and we can have any number.

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So this is the maximum flexibility, any number of elements.

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For authors we've also said

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we can have any number of authors

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that's in the actual database itself.

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Remember that our book store consists of a set of books and a set of authors.

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But we are going to specify

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something a little different for

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how many authors we have within a specific book.

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So let's continue to look

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at other cases where we've specified

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occurrence constraints.

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Here is the case where we're

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specifying how many authors we

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have within a book and

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again few boy this

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is a lot of XML here

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so take your time when looking

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at it; or for now just take my word for it.

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What we're specifying here is

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how many sub elements, how

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many auth sub elements we

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have within each author's element.

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And here we have no minOccurs

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specification only a maxOccurs.

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That means by default minOccurs is one.

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So what this is saying specifically, is

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that every book has in

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it's authors sub element, atleast

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one off, but we can have.

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any number of them, that's the string unbounded.

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Looking at the remaining occurrence constraints,

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for remarks, we have the

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minimum number of occurrences is zero.

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In other words, we don't have to have a remark.

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And we haven't specified max

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occurs so the default max occurs is one.

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So what we're saying here is that

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every book may have either

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no remark or exactly one

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remark but it may not

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have more than that.

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And there's a few more occurrence constraints that

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you can take a look at again

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as you browse the XML schema description on your own.

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Now let's make some changes in

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the document to test these occurrence constraints.

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So first let's remove the authors from our first book.

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We won't remove the whole author

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sub element but just the two

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off sub elements of authors.

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We attempt to validate and we see that it doesn't validate.

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We're missing some child elements,

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specifically the off-child elements

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because we expected there to be at least one of them.

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Incidentally, if we took

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the entire author sub-element out,

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we'll also get an error since

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we've specified the books must have author sub element.

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So now we're missing the entire

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author structure in that book and again we don't validate.

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Let's put authors back; and

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now let's look at the

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remark occurrence constraint so

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we said that every book can

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have zero or one

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remarks, so let's just add another remark to this book.

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00:12:34,710 --> 00:12:37,750
Oh, hi, actually remarks are allowed to be empty.

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00:12:38,770 --> 00:12:40,230
In any case, we have added a small remark.

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We validate and we see

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that we have too many remarks

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again because we specified that

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every book can have at most one remark.

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So that concludes our

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demonstration of XML schema again,

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it's been rather cursory we've

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only covered a few of

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the constructs but I did

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focus on the constructs that

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we have in XML schema

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that are not specifiable in DTDs.

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Finally one more time I

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urge you download the access

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fee and the document and play around with it yourself