Question

For me the scenario is hard to explain, but I would like to get a list of users (actually “patients”) which are ready for a dismissal meeting.

THE SITUATION

I have a patient table:

PATIENT
id   (int)
name (varchar)

The patient follows a special process, where I have defined phases:

PHASE
id   (int)
name (varchar)

And I log all the phases of the patient:

STATUS
id         (int)
phase_id   (int)
patient_id (int)
timestamp  (datetime)

The term “ready for a meeting” means the meeting is planned. Such planned meetings are logged in a message table. The message for a planned meeting has a type “meeting_planned” and for example when the meeting is done, there is a new message “meeting_done”.

MESSAGE
id         (int)
patient_id (int)
type       (enum)
value      (varchar)
timestamp  (datetime)

MY QUERY

I am looking for a query now which I really cannot come up with. My logic is:

1. Select all patients;
2. Where the last status (based on timestamp) is from phase name “C” or “D”;
3. Where there is no message with the type meeting_planned.

Ad 2: the patient can be either in one of the two phases. When a patient is in phase A, B, E or F then a meeting cannot be planned. A nurse may decide to plan the meeting already when patient is in phase C, but if that did not happened, it will occur at phase D (D always follows C, it is A>B>C>D>E>F).

Ad 3: there might be other messages, but never with this type. There is a unique constaint here on type+patient_id. If a meeting has been planned and has happened, there are two messages (type 1 = meeting_planned, type 2 = meeting_done). If there hasn’t been a meeting planned, the type meeting_planned does not occur, but other messages might be set already.

WHAT HAVE I TRIED

I came up with this query now:

SELECT p.id, p.name, ph.name

FROM patient p

LEFT JOIN status s  ON p.id=s.patient_id
LEFT JOIN phase ph  ON ph.id=s.phase_id
LEFT JOIN message m ON p.id=m.patient_id

WHERE (ph.name='C' OR ph.name='D')
AND (ph.name != 'E')
AND (m.type != 'meeting_planned' OR m.type IS NULL)

MY QUESTIONS

In above query I have two things I cannot manage:

1. I have patients double in the list. Any patient in phase D also occurs as a patient in phase C (so double). I tried a SELECT DISTINCT but that didn’t work out
2. I now exclude phases based on their name, but I would like to order phases on the timestamp from the status and only look at the last status from there

Ad 1: it looks now like this:

ID  NAME  PHASE
1   John  C
2   Jane  C
3   Tom   C
4   Pete  C
5   Anna  C
1   John  D
2   Jane  D
3   Tom   D

And obviously I want this:

ID  NAME  PHASE
4   Pete  C
5   Anna  C
1   John  D
2   Jane  D
3   Tom   D

I now I cannot use GROUP BY here because I then have to use a COUNT or similar like grouping functions.

Is there any SQL guru here to help me out?

Answer 1

SELECT p.id, p.name, max(ph.name) as phase

FROM patient p

    LEFT JOIN status s  ON p.id=s.patient_id
    LEFT JOIN phase ph  ON ph.id=s.phase_id
    LEFT JOIN message m ON p.id=m.patient_id

WHERE (ph.name='C' OR ph.name='D')
    AND (m.type != 'meeting_planned' OR m.type IS NULL)
    AND p.id not in (
        select p.id
        from 
            patient p
            inner join
            status s on p.id = s.patient_id
            inner join
            phase ph on ph.id = s.phase_id
        where ph.name > 'D'
        ) s
group by p.id, p.name