Hello. I am currently investigating AQL and would like to know how the
following queries would be answered by a conformant AQL query engine. I
understand that the structure of AQL query results are not standardized yet in
the AQL specifications, so if the spec cannot give a definitive answer what
would Ocean's AQL query engine implementation do?
1. SELECT
o/data[at0001]/events[at0031]/data[at0003]/items[at0004]/value AS
PosturalChangeSystolic,
o/data[at0001]/events[at1004]/data[at0003]/items[at0004]/value AS
ParadoxSystolic
FROM
EHR
CONTAINS COMPOSITION c [openEHR-EHR-COMPOSITION.encounter.v1]
CONTAINS OBSERVATION o [openEHR-EHR-OBSERVATION.blood_pressure.v1]
WHERE
o/data[at0001]/events[at0031]/data[at0003]/items[at0004]/value/value >=
140 OR
o/data[at0001]/events[at1004]/data[at0003]/items[at0004]/value/value >=
140
This query attempts to find all Systolic readings for Paradox and Postural
Change blood pressure events where the Systolic reading for either is >= 140.
As there is a one-to-many relationship between a blood pressure observation and
both Paradox and Postural Change events, how should the query be processed? If
an observation has three Postural Change events where Systolic >= 140, and four
such Paradox events, would the query return twelve rows (using my relational
database thinking). Or would one row be returned, having two "lists" (with
three and four members respectively) of Systolic readings?
2. SELECT o
FROM
EHR
CONTAINS COMPOSITION c [openEHR-EHR-COMPOSITION.encounter.v1]
CONTAINS OBSERVATION o [openEHR-EHR-OBSERVATION.blood_pressure.v1]
WHERE
o/data[at0001]/events[at0031]/data[at0003]/items[at0004]/value/value >=
140 OR
o/data[at0001]/events[at1004]/data[at0003]/items[at0004]/value/value >=
140
This query is the same as the previous one, except that it returns the whole
observation. It seems to me that all readings should be returned, regardless of
their systolic values?
3. SELECT o
FROM
EHR
CONTAINS COMPOSITION c [openEHR-EHR-COMPOSITION.encounter.v1]
CONTAINS OBSERVATION o [openEHR-EHR-OBSERVATION.blood_pressure.v1]
WHERE
o/data[at0001]/events[at0031]/data[at0003]/items[at0004]/value/value >=
140 AND
o/data[at0001]/events[at1004]/data[at0003]/items[at0004]/value/value >=
140
This query is the same as the previous one, except that the OR has been changed
to an AND. It could be argued that only the Paradox and Postural Change events
with a Systolic reading >= 140 should be returned, but it could also be argued
that all readings should be returned, as the whole observation has been
selected.
The problem I have is how to treat queries which have in the WHERE clause a
path expression that traverses through a 1:n relationship. In trying to think
through the semantics of such queries, I come up with ambiguities. In a
relational query (i.e. SQL) the "equivalent" path expression would have to be
expressed as a join between tables in the FROM clause, thus removing the
ambiguities.
Am I missing something or are my concerns relevant? If so, how does the
spec/Ocean implementation address them?
Thanks,
John Ryan-Brown
The Australian e-Health Research Centre
CSIRO ICT Centre
Brisbane
