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    <p>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).</p> <p><strong>1. Pivot (SQL Server 2008)</strong></p> <pre><code> 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 </code></pre> <p><strong>2. over clause (Oracle) based on a union</strong></p> <pre><code> 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 </code></pre> <p><strong>3. over clause (SQL Server 2008) with join and aggregate</strong></p> <pre><code> 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 </code></pre> <p><strong>4. Calculate ratio (SQL Server 2008)</strong></p> <pre><code> 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 </code></pre> <p><strong>5. Partial Comparision of two (or more) tables (SQL Server 2008)</strong></p> <pre><code>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) &gt; 0.00000001 </code></pre> <p>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.</p>
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    1. POHow does a sub query in the from clause work?
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