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Left join of the same table but take 2 different columns .

Posted on 2014-01-24
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Last Modified: 2014-01-31
i have a table A , with columns A.A,A.B,A.C,A.D . For each matching row in the table my query should give 2 rows one row with A.A as original and one row with A.B as original , how can i accomplish this .
Resultset :
1. A.A as original,A.C,A.D
2. A.B as original,A.C,A.D
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Question by:FranklinRaj22
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Expert Comment

by:Jim Horn
ID: 39806940
I think we need to see some mockup data of what you're trying to pull off here, as the term 'original' isn't real intuitive.
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Author Comment

by:FranklinRaj22
ID: 39806962
Table A

ColA           ColB   ColC     ColD

America     USA     1          2
Russia        USSR    5         6

Result should be
Col1        Col2   Col3
America   1        2
USA          1        2
Russia      5       6
USSR         5       6

I am able to accomplish this with but want to know if there are any better ways to do it ...
(select ColA as col1,ColC as col2,ColD as col3 from A) UNION (select ColB as col1,ColC as col2,ColD as col3 from A)
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LVL 143

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 39807207
that is not a "left join", but a UNION ALL of the same table ..
select a.cola, a.colc, a.cold from tableA a
UNION ALL
select a.colb, a.colc, a.cold from tableA a

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LVL 143

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 39807214
note: UNION should be avoided whenever you can, and use UNION ALL instead.
UNION is performance an implicit DISTINCT over the resulting outcome, which usually is not required, and hence just wasting resources.
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Accepted Solution

by:
Scott Pletcher earned 1500 total points
ID: 39807599
No need to scan the table twice; instead, use a CROSS JOIN.  You could also use a CROSS APPLY.:

SELECT
    CASE WHEN whichCol = 'A' THEN A.ColA ELSE A.ColB END AS Col1,
    A.ColC,
    A.ColD
FROM dbo.tablename A
CROSS JOIN (
    SELECT 'A' AS whichCol UNION ALL
    SELECT 'B'
) AS whichCols
0

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