Question

I’ve got a visits table that looks like this:

id identity(1,1) not null,
visit_date datetime not null,
patient_id int not null,
flag bit not null

For each record, I need to find a matching record that is same time or earlier, has the same patient_id, and has flag set to 1. What I am doing now is:

select parent.id as parent_id, 
(
   select top 1 
      child.id as child_id
   from 
      visits as child
   where 
     child.visit_date <= parent.visit_date
     and child.patient_id = parent.patient_id
     and child.flag = 1
   order by 
     visit_date desc
 ) as child_id
from 
   visits as parent

So, this query works correctly, except that it runs too slow — I suspect that this is because of the subquery. Is it possible to rewrite it as a joined query?

Answer 1

   /* 
    id identity(1,1) not null,
    visit_date datetime not null,
    patient_id int not null,
    flag bit not null
   */

SELECT
    T.parentId,
    T.patientId,
    V.id AS childId
FROM
    (
    SELECT
        visit.id AS parentId,
        visit.patient_id AS patientId,
        MAX (previous_visit.visit_date) previousVisitDate
    FROM
        visit
        LEFT JOIN visit previousVisit ON
            visit.patient_id = previousVisit.patient_id
            AND visit.visit_date >= previousVisit.visit_date
            AND visit.id <> previousVisit.id
            AND previousVisit.flag = 1
    GROUP BY
        visit.id,
        visit.visit_date,
        visit.patient_id,
        visit.flag
    ) AS T
    LEFT JOIN visit V ON
      T.patientId = V.patient_id
      AND T.previousVisitDate = V.visit_date