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Union or Union All

Posted on 2009-06-30
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Last Modified: 2012-05-07
Which query should I use if I don't want duplicates:
  Union or Union All
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Question by:schmir1
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7 Comments
 
LVL 17

Expert Comment

by:pssandhu
ID: 24746172
Union All
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LVL 75

Assisted Solution

by:Aneesh Retnakaran
Aneesh Retnakaran earned 400 total points
ID: 24746174
Union wont return the duplicates

Union all returns everything
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LVL 17

Assisted Solution

by:pssandhu
pssandhu earned 400 total points
ID: 24746188
Oops... my bad, I though you wanted duplicates
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Guy Hengel [angelIII / a3] earned 800 total points
ID: 24746333
> if I don't want duplicates:
it depends on what you define a "duplicate".

union will do a distinct, but only over ALL the columns returned by the query.
if your "duplicates" are in a single (or part of the ) column(s), neither will work, but you will need to change the query eventually completely.
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LVL 14

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by:shru_0409
shru_0409 earned 400 total points
ID: 24746361
The UNION clause forces all rows returned by each portion of the UNION to be sorted and merged and duplicates to be filtered before the first row is returned to the calling module.

A UNION ALL simply returns all rows including duplicates and does not have to perform any sort, merge, or filter.
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LVL 32

Expert Comment

by:awking00
ID: 24748095
Just so you can see the difference, see the attached. Notice how the union results are ordered and the union all results are just stacked.
union-queries.txt
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Author Closing Comment

by:schmir1
ID: 31598378
Thanks for all your help.  I think I have a good understanding of Union now.
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