SQL MAX query using 3 tables

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 seek 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'