Help on XQuery Retrieval

Question

I'm having a problem understanding the results of a Return in a FLWOR. Basically I want the second part of the XQuery to operate on the total return of the first part.
I have a series of documents in one collection. Each document at the top level has the tag <flight-data> which has an aircraft ID as an attribute. Then within the document, there is lower level data under the tag <maintenance-raw-data> that contains fault IDs.
An example of two of the documents in the collection:
<flight-data aircraft-id = “1”>
<maintenance-raw-data fault-id=“0001”>
<maintenance-raw-data fault-id=“0002”>
<maintenance-raw-data fault-id=“0003”>
<maintenance-raw-data fault-id=“0001”>
</flight-data>
<flight-data aircraft-id = “2”>
<maintenance-raw-data fault-id=“0002”>
<maintenance-raw-data fault-id=“0003”>
<maintenance-raw-data fault-id=“0003”>
<maintenance-raw-data fault-id=“0004”>
<maintenance-raw-data fault-id=“0005”>
</flight-data>
So the intent of the xquery was to select all the data for one or more aircraft, then on that “total” selection, get a distinct list of all fault IDs. The xquery is as follows:
for $z in collection("brickdata2")/flight-data
where $z/@aircraft-id="1" or $z/@aircraft-id="2") return
for $x in distinct-values($z//maintenance-raw-data/@fault-id) return $x
Basically, I want the query to return the flight-data for both aircraft and then, on that total return, find the unique fault IDs. However, the second part operates on the unique fault IDs for each aircraft separately. It gives the unique fault IDs for the first aircraft and then the unique fault IDs for the second aircraft.
Query results with Aircraft 1 and Aircraft 2 in Where clause
Fault ID 0001
Fault ID 0002
Fault ID 0003
Fault ID 0002
Fault ID 0003
Fault ID 0004
Fault ID 0005
So we then changed the first line of the XQuery to “let $z := collection”. etc.” and then the query retrieving both aircraft works (you get 0001, 0002, 0003, 0004, 0005) “ but if you change the where clause to retrieve for only one aircraft (either one), you get the same answer (i.e., fault IDs for both aircraft).
Appreciate any help. Thanks in advance.

Answer 1

Basically, I want the query to return the flight-data for both aircraft and then, on that total return, find the unique fault IDs. However, the second part operates on the unique fault IDs for each aircraft separately. It gives the unique fault IDs for the first aircraft and then the unique fault IDs for the second aircraft.You can do it like the following, by first retrieving the whole sequence of attributes for both aircrafts, then applying distinct-values() :
distinct-values(
  for $z in collection("brickdata2")/flight-data
  where $z/@aircraft-id = "1"
     or $z/@aircraft-id = "2"
  return $z/maintenance-raw-data/@fault-id
)