XQuery FLWR Expressions

(Q9) List the titles of books published by Morgan Kaufmann in 1998.

FOR $b IN document("bib.xml")//book
WHERE $b/publisher = "Morgan Kaufmann"
    AND $b/year = "1998"
RETURN $b/title

(Q10) List each publisher and the average price of its books.

FOR $p IN distinct(document("bib.xml")//publisher)
LET $a := avg(document("bib.xml")
    /book[publisher = $p]/price)

RETURN
    <publisher> 
        <name> $p/text() </name> , 
        <avgprice> $a </avgprice> 
    </publisher>