Selecting Objects Based on Criteria (Filtering)

To select a sequence of objects that satisfy some criteria, we can use a predicate expression to filter the sequence:

let $books := (
  {
    "title" : "A Brief History of Time",
    "author" : "Hawking, Stephen"
  },
  {
    "title" : "Your Inner Fish",
    "author" : "Shubin, Neil"
  }
)
return $books[ contains( $$.author, "Shubin" ) ]

{
  "title" : "Your Inner Fish", 
  "author" : "Shubin, Neil"
}

The use of [ ] and the expression inside is the predicate expression that means in this case: the subset of books where, for each book denoted by $$, the author contains Shubin. The $$ is known as the context item (each item in the sequence). The use of [ ] and $$ is a short-hand way of writing:

for $book in $books
where contains( $book.author, "Shubin" )
return $book

Note that in XQuery, the . (dot) refers to the context item; in JSONiq, however, the . is used for object key selection, so $$ is used for the context item instead.