rhservan
asked on
Is this a correct use of UNION?
When using a UNION to eliminate duplicates, will it work when using count as in columnB below?
ColumnA, count(distinct columnB)
From TableA
UNION
ColumnA, count(distinct columnB)
From TableA
ColumnA, count(distinct columnB)
From TableA
UNION
ColumnA, count(distinct columnB)
From TableA
select
ColumnA, count(distinct columnB)
From TableA
group by ColumnA
is this what you need?
ColumnA, count(distinct columnB)
From TableA
group by ColumnA
is this what you need?
yes it will, but you can re-write as below
ColumnA, count(distinct columnB)
From TableA group by ColumnA
UNION
ColumnA, count(distinct columnB)
From TableA group by ColumnA
but it remains pointless... I'm not following (or my eyes are deceiving me)
let's say the upper query produces this
ABCD 1
DEFG 3
and the lower produces exactly the same, together when "stacked" and sorted would be:
ABCD 1
ABCD 1
DEFG 3
DEFG 3
the UNION then looks for repeats, and the end result is:
ABCD 1
DEFG 3
i.e. equivalent to the upper query
Note, UNION considers the "whole row"
let's say the upper query produces this
ABCD 1
DEFG 3
and the lower produces exactly the same, together when "stacked" and sorted would be:
ABCD 1
ABCD 1
DEFG 3
DEFG 3
the UNION then looks for repeats, and the end result is:
ABCD 1
DEFG 3
i.e. equivalent to the upper query
Note, UNION considers the "whole row"
rhservan, this is a weird question, it looks like you're not mentioning a crucial part of the story. As Paul already pointed out, query A and query 2 are exactly the same and thus produce the same result, which is then "undone" by the UNION.
Or are you actually trying to duplicate your data? In that case you can use UNION ALL.
Or are you actually trying to duplicate your data? In that case you can use UNION ALL.
ASKER
I'm not sure what I am missing:
If ColumnB in both queries are based on a count, not a list, does the UNION still recognize
duplication of the column being counted?
If ColumnB in both queries are based on a count, not a list, does the UNION still recognize
duplication of the column being counted?
ASKER CERTIFIED SOLUTION
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maybe if I add this it will help...
making rows unique by UNION occurs AFTER each of the subqueries are performed, union therefore only considers the results (not how you got to the results)
making rows unique by UNION occurs AFTER each of the subqueries are performed, union therefore only considers the results (not how you got to the results)
Perhaps we can help you better if you explain a bit more what you're actually trying to achieve with that query... Show us a couple of data examples.
ASKER
PP this is exactly what I was looking for:
"What UNION will NOT understand is "duplication of the column being counted" because it will ONLY assess the RESULT of the count.
if you need to remove "duplication of the column being counted" you must do that before counting them."
How can I accomplish this?
"What UNION will NOT understand is "duplication of the column being counted" because it will ONLY assess the RESULT of the count.
if you need to remove "duplication of the column being counted" you must do that before counting them."
How can I accomplish this?
ASKER
You mean, logically, something like this would work:
ColumnA, ColumnB , count(distinct columnB)
From TableA
Group By ColumnA, ColumnB
UNION
ColumnA, ColumnB , count(distinct columnB)
From TableA
Group By ColumnA, ColumnB
ColumnA, ColumnB , count(distinct columnB)
From TableA
Group By ColumnA, ColumnB
UNION
ColumnA, ColumnB , count(distinct columnB)
From TableA
Group By ColumnA, ColumnB
both the upper and lower queries would produce the same results, so the union will disgard one row
you might as well only do the upper query.
I suspect there's more here