SQL MAX query using 3 tables - sql

Question

I'm having difficulty querying an SQL database.
| patient_id | episode_number | attend_practitioner | pract_assignment_date |
| ---------- | -------------- | ------------------- | --------------------- |
| 111 | 4 | 4444 | 01/05/2017 |
| 222 | 8 | 5555 | 03/17/2017 |
| 222 | 8 | 6666 | 03/20/2017 |
| 222 | 9 | 7777 | 04/10/2017 |
| 333 | 2 | 5555 | 10/08/2017 |
| 444 | 7 | 7777 | 08/09/2017 |
| patient_id | episode_number | backup_practitioner | date_of_assignment |
| ---------- | -------------- | ------------------- | ------------------ |
| 111 | 4 | | |
| 222 | 8 | 7777 | 03/17/2017 |
| 222 | 8 | 4444 | 05/18/2017 |
| 222 | 9 | | |
| 333 | 2 | 4444 | 10/08/2017 |
| 333 | 2 | 5555 | 10/19/2017 |
| patient_id | episode_number | admit_date |
| ---------- | -------------- | ---------- |
| 111 | 4 | 01/05/2017 |
| 222 | 8 | 03/17/2017 |
| 222 | 9 | 03/20/2017 |
| 333 | 2 | 10/08/2017 |
I'm looking for an SQL query where I can enter a staff_id and then have it return all the open episodes they are currently assigned to. Result:
| staff_id | patient_id | episode_number | admit_date | date_of_assignment |
| -------- | ---------- | -------------- | ---------- | ------------------ |
| 4444 | 111 | 4 | 01/05/2017 | 01/05/2017 |
| 4444 | 222 | 8 | 03/17/2017 | 05/18/2017 |
I don't understand how the SQL handles aliases in the query.
The SQL doesn't know what to do with SQL window functions such as OVER, LAG(), LEAD(), etc. So I'm using self joins along with the MAX() function. Maybe this is an older SAP SQL server.
I don't know whether capitalization in the SQL query is irrelevant.

Answer 1

This is a bit hard to work out from your question. I thin you need to most recent assignment for each patient/episode.
You could do this with a second sub-query using max, or you could use not exists as per below.
As you mentioned, it would be much easier with a row_number() and or CTE's, but this is bulk standard sql version.
select s.staff_id, e.patient_id, e.episode_number as episode_id, e.admit_date, s.date_of_assignment, c.last_date_of_service
from episode_history e
join (
select patient_id, episode_number, attend_practitioner as staff_id, pract_assignment_date as date_of_assignment
from history_attending_practitioner
union
select patient_id, episode_number, backup_practitioner as staff_id, date_of_assignment as date_of_assignment
from user_practitioner_assignment
where backup_practitioner is not null
) s on s.patient_id=e.patient_id and s.episode_number=e.episode_number
and not exists(select * from history_attending_practitioner a2 where a2.patient_id=s.patient_id and a2.episode_number=s.episode_number and a2.pract_assignment_date>s.date_of_assignment)
and not exists(select * from user_practitioner_assignment a3 where a3.patient_id=s.patient_id and a3.episode_number=s.episode_number and a3.date_of_assignment>s.date_of_assignment)
join view_episode_summary_current c on c.patient_id=e.patient_id and c.episode_number=e.episode_number
where e.discharge_date is null

Answer 2

After tweaking the query items, this is the SQL query version that returned the correct results:
SELECT
t1.staff_id
, t1.patient_id
, t3.admit_date
, t1.episode_number
, t1.date_of_assignment
FROM
/* select the most recent attending practitioner entry from table1 for the patient and episode */
(SELECT attending_practitioner AS staff_id, patient_id, episode_number, pract_assignment_date AS date_of_assignment
FROM table1 AS t1a
WHERE t1a.pract_assignment_date =
(SELECT MAX(pract_assignment_date)
FROM table1 AS t1b
WHERE t1b.patient_id = t1a.patient_id
AND t1b.episode_number = t1a.episode_number)
UNION
/* select the most recent practitioner entry from table2 for the patient and episode */
SELECT backup_practitioner AS staff_id, patient_id, episode_number, date_of_assignment
FROM table2 AS t2a
WHERE t2a.date_of_assignment =
(SELECT MAX(date_of_assignment)
FROM table2 AS t2b
WHERE t2b.patient_id = t2a.patient_id
AND t2b.episode_number = t2a.episode_number)
) AS t1
INNER JOIN
/* filter out closed episodes by using the table3 */
(SELECT patient_id AS patient_id2, episode_number AS episode_number2, admit_date
FROM table3) AS t3
ON t3.patient_id = t1.patient_id
AND t3.episode_number2 = t1.episode_number
WHERE t1.staff_id = '4444'