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This video gives an introduction to

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the XQuery query language for XML.

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As a reminder, querying XML

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is not nearly as mature as querying relational databases.

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It's much newer.

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And there is no clean underlying algebra

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that is equivalent to the relational algebra.

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The sequence of development of XML

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querying which is started with

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XPath, which we've already covered.

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Then Access LT, which we'll

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be covering later; and in this

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video and the subsequent demo,

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we'll be covering the XQuery language.

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XQuery is an expression language,

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also known as a compositional language.

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And we've seen that already with relational algebra.

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What that means is, when we

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run an expression of the language

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on a type of data the

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answer to that expression or query is the same type of data.

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So let me just draw the picture here.

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We have some data.

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We run an expression or

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query over that data

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and we get get a result back

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and that result is also going

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to be in the same type of data.

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That means we can run another

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query or expression over

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the same type of data

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that was the result of the previous one.

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And again, we'll again, we get a result of the same type of data.

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Now that also means that we can sort of put these together.And

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when we put them together, that

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will give us additional expressions or

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queries that are valid in the language.

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When we talked about compositionality in

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the relational model, the types

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of data we were using were relations.

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We would have maybe some relations

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here, we run a query over

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those relations the result itself

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is a relation and we can

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run further queries over that

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relation that's the answer.

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And then when we looked at relational

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algebra, we saw a

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composing the expressions of relational

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algebra into more complicated

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expressions and everything worked together.

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In XML, it's similar, except the

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expressions operate on and return

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what are known as sequences of elements.

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Now we're not going to go into a great detail of this model.

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As we mentioned for X Path,

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the sequences of elements can be generated from an XML document.They

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can also be generated from an

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XML stream and the

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interpretation is quite natural.

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Now let me mention that one

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of the basic types of expressions in XQuery is XPath.

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So every XPath expression

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is in fact an XQuery

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expression or a valid query in XQuery.

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Then we can use XPath and

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other constructs of X query

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to put together into more complicated queries.

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Now, one of the most commonly used

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expressions in XQuery is

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what's known as the FLWOR expression,

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and it looks closest to sequel

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of any of the constructs in X query.

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Let me explain how it works.

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We have up to five clauses in

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that FLWOR expression and that's

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where the FLWOR comes from.

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The first, the for clause,

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sets up what are known as iterator variables.

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So the idea here is that

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in the for clause, this expression

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is evaluated and it will

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produce potentially a set result.

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And then the variable will

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be backed Count to each element

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of the set one at a

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time and then the

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rest of the expression will be evaluated for each element.

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So in other words if this produces a set of end results.

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Then we will effectively evaluate the rest of the queries N times.

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We will see that much more clearly when we do the demonstration.

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The let clause is more of a typical assignment.

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So it's only run once, each time the rest of the query is run.

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And so this expression is evaluated

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and even if it's a set, it's

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assigned once to this variable.

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So it's not iterating now, it's just doing an assignment.

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Again, it'll become quite clear when we do the demonstration.

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The where clause specifies a

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condition and it's a filter very

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similar to the filters that we've seen in SQL.

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The order by is also sort of similar to SQL.

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It sorts the results so

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that you can get the result in a particular order.

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And finally the return clause says

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what we want to actually get in the result of our query.

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And just as a reminder, when we

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start the query with the

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for, if we are effectively

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executing the query N times

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then each one of those

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N executions may result

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in one of the elements in the result.

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The FLWOR expression has up

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to five clauses, but actually

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all of the clauses except the return clause are optional.

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The return is the only one

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that's required to have a well-formed query and to get a result.

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I also want to mention that

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the for and let clause can

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be repeated multiple times if

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we want to set up multiple

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variables or multiple assignments and they can be interleaved with each other.

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They don't need to be in a particular order.

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A next query, it's possible to

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mix query evaluation with

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simply writing down the XML data that we want in the result.

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And here's an example.

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In this case, we're writing the

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opening and closing tags that

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we want to be in the result of our query.

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And then inside curly braces,

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we write the query itself.

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And what these curly braces

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effectively say are evaluate me.

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The curly braces are an

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indicator to the XQuery processor

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that what's inside them needs to

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be run as a query,

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is replaced with the XML that's the query result,

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and the final output is

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our opening and closing tags with

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the XML query result inside.

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At this point, let's move ahead to the demo.

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We'll once again be using our

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bookstore data, and we'll do

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a sequence of XQuery examples over that data.

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Again we are not going to be comprehensive.

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XQuery is a big

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language but will show the

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flavor of the language and a

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lot of its most useful constructs and

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we'll be able to write some quite powerful queries.