XQuery: difference between let and for

Question

The output of

<output>{
  let $m := 2
  let $n := "Hello"
  let $s := $m+1
  return (
    <m>{ $m }</m>,
    <n>{ $n }</n>,
    <s>{ $s }</s>
  )
}</output>

is

<output>
  <m>2</m>
  <n>Hello</n>
  <s>3</s>
</output>

Why is the output for the following query identical?

<output>{
  for $m in 2
  for $n in "Hello"
  for $s in $m+1
  return (
    <m>{ $m }</m>,
    <n>{ $n }</n>,
    <s>{ $s }</s>
  )
}</output>

Christian Grün Christian Grün · Accepted Answer · 2018-07-21T06:08:31

If the expression that is bound to for yields a single item as result (such as in your example), it is equivalent to using let. If it returns multiple items, things look different:

With for, each item will be bound to the variable one by one.
With let, all items (i.e., the full sequence) will be bound once.

You will notice the difference if you e.g. replace 2 by (1,2,3) in your query and run it again.

XQuery: difference between let and for

1 Answers