これらのクエリはどちらも同じことを行い、一方は他方の約 2.5 倍の速度で実行されます。
遅いクエリ
select sc.countyName
,(
--this adds the distinct number of people who received rehab
select count(distinct p2.patientid)
from statecounties as sc2
inner join Patient as p2 on p2.stateCode=sc2.stateCode
and p2.countyCode = sc2.countyCode
and sc2.stateCode='21'
inner join Claims as c on c.patientid=p2.patientid
and c.admissiondate = p2.admissiondate
and c.dischargedate = p2.dischargedate
where sc2.countyName=sc.countyname and c.hcpcs in ('97001', '9339', '97002')
) as rehabClaims
--this is the total number of people from each county that visited a hospital with a DRG stroke admission
,count(*) as visitCounts
,(
--this adds the distinct number of people who received rehab
select count(distinct p2.patientid)
from statecounties as sc2
inner join Patient as p2 on p2.stateCode=sc2.stateCode
and p2.countyCode = sc2.countyCode
and sc2.stateCode='21'
inner join Claims as c on c.patientid=p2.patientid
and c.admissiondate = p2.admissiondate
and c.dischargedate = p2.dischargedate
where sc2.countyName=sc.countyname and c.hcpcs in ('97001', '9339', '97002')
)/count(*) as visitCounts
from StateCounties as sc
inner join Patient as p on p.stateCode=sc.stateCode
and p.countyCode=sc.countyCode
where sc.stateCode='21'
group by sc.countyName
having count(*) > 30
高速クエリ
select *, round(cast(rehabclaims as float)/VisitCounts *100,2) as percentRehab
from
(
select sc.countyName
,(
--this adds the distinct number of people who received rehab
select count(distinct p2.patientid)
from statecounties as sc2
inner join Patient as p2 on p2.stateCode=sc2.stateCode
and p2.countyCode = sc2.countyCode
and sc2.stateCode='21'
inner join Claims as c on c.patientid=p2.patientid
and c.admissiondate = p2.admissiondate
and c.dischargedate = p2.dischargedate
where sc2.countyName=sc.countyname and c.hcpcs in ('97001', '9339', '97002')
) as rehabClaims
--this is the total number of people from each county that visited a hospital with a DRG stroke admission
,count(*) as visitCounts
from StateCounties as sc
inner join Patient as p on p.stateCode=sc.stateCode
and p.countyCode=sc.countyCode
where sc.stateCode='21'
group by sc.countyName
having count(*) > 30
) t
主な違いは、最初の除算はsub selectsそれ自体で行われるのに対し、高速なクエリではサブクエリを使用することです (専門用語が間違っている場合はお詫びします)。式全体を入力するよりも columnName/otherColumnName を読む方がはるかに簡単なので、私は 2 番目の方法を好みます。これも非常に高速な場合が多いようです。これには理由がありますか、それとも単なる逸話ですか (適切なインデックスが適用されます)