How does a sub query in the from clause work? - sql

Question

What is a valid use-case for Sub Query in the FROM clause? How does that scheme work? There are many examples of this type in SO. A link to one of those is here but I can't see how this scheme works.
P.S: If the answer is Oracle specific it is fine.

Answer 1

Here are some use cases for a subquery in the from clause. How it works has been explained in the comments to your question (SQL is mathematical closed thanks to its relational operators).
1. Pivot (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. over clause (Oracle) based on a union
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. over clause (SQL Server 2008) with join and aggregate
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. Calculate ratio (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. Partial Comparision of two (or more) tables (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
Note: These are examples only and in I thought that the subquery in the from clause have been a good solution to achieve the result.