是否可以从连接中的子查询结果中得到 select

Question

So I have a large subquery and I would like to join on that subquery while using the result of the subquery in the join.

For example, I have a table called patient and one called appointment , and I would like to get the number of appointments per patient with given criteria.

Right now I am doing something like this:

SELECT
  t1.*
FROM
  (
    SELECT
      patient.name,
      patient.id,
      appointment.date
    FROM
      patient
      LEFT JOIN appointment ON appointment.patient_id = patient.id
    WHERE
      /* a **lot** of filters, additional joins, etc*/
  ) t1
  LEFT JOIN (
    SELECT
      COUNT(*) number_of_appointments,
      patient.id
    FROM
      patient
      LEFT JOIN appointment ON appointment.patient_id = patient.id
    GROUP BY
      patient.id
  ) t2 ON t1.id = t2.id

The problem is that this returns the number of appointments for each patient independent from the subquery above it. I tried writing the join as this:

LEFT JOIN (
    SELECT
      COUNT(*) number_of_appointments,
      patient.id
    FROM
      t1
    GROUP BY
      patient.id
  )

But obviously I'm getting an error saying that table t1 doesn't exist. Is there any way for me to do this cleanly without having to repeat all of the filters from t1 in t2?

Thanks!

Answer 1

Why not use window functions?

SELECT p.name, p.id, a.date,
       COUNT(a.patient_id) OVER (PARTITION BY p.id) as num_appointments
FROM patient p LEFT JOIN
     appointment a
     ON a.patient_id = p.id
WHERE . . .

This provides the count based on the WHERE filtering. If you wanted a count of all appointments, then do the calculation before applying the WHERE :

SELECT p.name, p.id, a.date,
       COALESCE(a.cnt, 0) as num_total_appointments,
       COUNT(a.patient_id) OVER (PARTITION BY p.id) as num_matching appointments
FROM patient p LEFT JOIN
     (SELECT a.*,
             COUNT(*) OVER (PARTITION BY a.patient_id) as cnt
      FROM appointment a
     ) a
     ON a.patient_id = p.id
WHERE . . .