使用条件优化 Postgres 查询

Question

I have a query where I am joining tables and checking if the particular incident has failed a processing with error kode 76,75,10 and checking another condition where i checked if course has only 1 incident.

Is there any way to check if course has only 1 incident then only the 1st condition happens in a single query??

Tried with case --> when but not sure

select * from course c where id in(select i.course_id from incident i
 join report r on i.report_id=r.id
 join error e on e.report_id=r.id
 join error_code ek on e.error_code_id=ek.id where errorcode in (76,75,10)) and
     (select count(*) from incident i where i.course_id = c.id) =1

Answer 1

If both conditions in and are fulfilled, then the single incident of course must be of errorcode 76,75 or 10. So take all incidents with that errorcodes and return only those courses for which there is just one such incident.

select * from course c where id in (
  select i.course_id
  from incident i
  join ... -- r,e,ek on obvious conditions
  where errorcode in (76,75,10)
  group by i.course_id
  having count(distinct i.id) = 1
)

(I used distinct since it isn't clear what kind are the relations i->r->e->ek of. If all of them are many to one, then distinct isn't necessary, otherwise it must be there to avoid false-duplicate i.id s due to joins.)