FROM 句でのサブクエリの有効な使用例は何ですか? そのスキームはどのように機能しますか?SOにはこのタイプの例がたくさんあります。それらの 1 つへのリンクはここにありますが、このスキームがどのように機能するかわかりません。
PS: 答えが Oracle 固有のものであれば問題ありません。
from 句でのサブクエリの使用例を次に示します。それがどのように機能するかは、質問へのコメントで説明されています(SQLは関係演算子のおかげで数学的に閉じています)。
1. ピボット (SQL Server 2008)
select P.RUN_ID
, [2012] = sum(P.[2012])
, [2013] = sum(P.[2013])
, [2014] = sum(P.[2014])
, [2015] = sum(P.[2015])
from (select T.RUN_ID
, Y.YEAR
, T.MEASURE
from SOME_TABLE T
inner join
YEAR Y
on T.SOME_ID = Y.SOME_ID
) T
pivot (
sum(MEASURE)
for YEAR in ([2012], [2013], [2014], [2015])
) P
group by
P.RUN_ID
order by
P.RUN_ID
2. 結合に基づく over 句 (Oracle)
select S.Text_ID
, row_number() over (partition by S.Text_ID order by S.Segmentstart) as Segmentnumber
, S.Segment_ID
, S.Segmentstart
, S.Segmentend
, S.Segmentfragment
from (select S.Text_ID as Text_ID
, S.Satz_ID as Segment_ID
, S.Start as Segmentstart
, S.End as Segmentend
, S.Fragment as Segmentfragment
from Mainclauses S
union all
select X.ID as Text_ID
, null as Segment_ID
, coalesce(S.End, 0) as Segmentstart
, lead(S.Start, 1, X.CONTENT_LENGTH) over (partition by X.ID order by S.Start) as Segmentend
, 'X' as Segmentfragment
from Texts X
left join
Mainclauses S
on X.ID = S.Text_ID
union all
select X.ID as Text_ID
, null as Segment_ID
, 0 as Segmentstart
, min(S.Start) as Segmentend
, 'X' as Segmentfragment
from Texts X
inner join
Mainclauses S
on X.ID = S.Text_ID
group by
X.ID
) S
3. 結合と集計を含む over 句 (SQL Server 2008)
select E.X_ID
, Z.SomeThing
, sum(Z.OtherMeasure * E.Measure) as CombinedMeasure
, Sorting = row_number() over
( partition by
E.X_ID
order by
Z.SomeThing
)
from (select E.X_ID
, E.Y_ID
, Measure = sum(E.Measure)
from SomeTable E
group by
E.X_ID
, E.Y_ID
) E
inner join
OtherTable Z
on E.Y_ID = Z.Y_ID
4. 比率を計算する (SQL Server 2008)
with SomeData
( Main_ID
, Sub_ID
, Measure
)
as (select Main_ID
, Sub_ID
, Measure = sum(Measure)
from SomeTable P
group by
Main_ID
, Sub_ID
)
select Main_ID
, Sub_ID
, Ratio = D.Measure / sum(M.Measure) over (partition by M.Main_ID)
from SomeData D
inner join
(select Main_ID
, Measure = sum(Measure)
from SomeData
group by
Main_ID
having sum(Measure) != 0
) M
on M.Main_ID = D.Main_ID
5. 2 つ (またはそれ以上) のテーブルの部分比較 (SQL Server 2008)
select *
from (select A, M = sum(M) from S group by A) X
full outer join
(select A, M = sum(M) from T group by A) Y
on X.A = Y.A
where X.A is null
or Y.A is null
or abs(X.M - Y.M) > 0.00000001
注: これらは単なる例であり、from 句のサブクエリは結果を達成するための優れたソリューションであると考えました。