xml - XQuery difference between "where" on a "for" clause and "where" on a "let" clause -

consider following xml:

<bib> <book year="1994">     <title>tcp/ip illustrated</title>     <author><last>stevens</last><first>w.</first></author> <publisher>addison-wesley</publisher> <price>65.95</price> </book>  <book year="2000">     <title>data on web</title>     <author><last>abiteboul</last><first>serge</first></author>     <author><last>buneman</last><first>peter</first></author>     <author><last>suciu</last><first>dan</first></author>     <publisher>morgan kaufmann publishers</publisher>     <price>39.95</price> </book>  <book year="1999">     <title>the economics of technology , content digital tv</title>     <editor><last>gerbarg</last><first>darcy</first><affiliation>citi</affiliation></editor>             <publisher>kluwer academic publishers</publisher>     <price>129.95</price> </book> </bib> 

i know following xquery query

for $book in doc(“books.xml”)//book $book/price>60 return <expensivebook> { $book/title/text() } </expensivebook> 

will return title of 2 books price greater 60:

<expensivebook>tcp/ip illustrated</expensivebook> <expensivebook>the economics of technology , content digital tv</expensivebook> 

instead following query:

let $books := doc(“books.xml”)//book $books/price > 60 (: existential... :) return <expensivebooks> { $books/title } </expensivebooks> 

will return whole of 3 books in xml doc:

<expensivebooks>     <title>tcp/ip illustrated</title>     <title>data on web</title>     <title>the economics of technology , content digital tv</title> </expensivebooks> 

i can't understand why happens, shouldn't have whole of books variable declared "let" , filter clause? why returns "data on web" book price less 60?

another curious thing can't understand query following one:

let $books in doc(“books.xml”)//book[price>60] return <expensivebook> { $books/title } </expensivebook> 

correctly returns 2 books price greater 60:

<expensivebooks>     <title>tcp/ip illustrated</title>     <title>the economics of technology , content digital tv</title> </expensivebooks> 

how things work here? why behaviour isn't 1 expected? clause work differently on let , clauses? , last query, "let" clause used specifing price element through xpath syntax [element=value]?

thanks help, :)

i think need find basic resources on xpath , xquery. when say:

let $books := doc(“books.xml”)//book $books/price > 60 (: existential... :) return <expensivebooks> { $books/title } </expensivebooks> 

what means is: bind $books sequence of book elements, consider sequence of price elements of books in sequence: if of them more 60, return sequence of title elements of of book elements in $books, wrapped in element. or none.

your first query said: iterate through sequence of book elements, , if 1 have right has price more 60, return title.

the query constraint on xpath:

let $books in doc(“books.xml”)//book[price>60] return <expensivebook> { $books/title } </expensivebook> 

says: construct sequence of book elements price child element more 60, , return element consisting of sequence of titles of books, wrapped in element.

let binds variable; binds variable each member of sequence in turn. path expressions on sequences produce other sequences. '=' , '>' existential operators operate on sequences , return single boolean result.