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Re: Ambiguous column names in subselects - how resolve?

From: <>
Date: Tue, 18 Sep 2007 06:20:31 -0700
Message-ID: <>

On Sep 17, 9:55 pm, (Malcolm Dew-Jones) wrote:
> ( wrote:
> : I wouldn't be including 'counter' in my result set since you're using
> : it as a limiting condition on the number of rows returned, [...]
> : SELECT a, b, c
> : FROM (
> : SELECT ta.a, tb.b, ta.c, ROWNUM as counter
> : FROM A ta, B tb
> : WHERE tb.c = ta.c -- proper join condition here, else cartesian
> : product
> : ORDER BY ta.a
> : )
> : WHERE counter <= 25;
> As a general rule, I have to disagree with that.
> Columns that are used for filtering should be left in the result set
> unless there is a need to remove them. This makes is easier for the
> original developer and subsequent maintainers to confirm the query is
> selecting the anticipated data (i.e. it is easier to confirm the query is
> correct).
> Leaving the limiting data in the result set also makes it easier for a
> maintenance programmer to examine the data when creating ways to test any
> changes that are required.
> Consider a query that has an over all structure like
> select columns,including,limiters
> from table(s)
> where
> join conditions
> and major category conditions
> and specific conditions such as ID=:the_id
> This query can be run with three different levels of specificity by cut
> and paste with no edits.
> 1 Cut and paste including just the join condition and you see all
> available data to confirm the join condition is correct (null data?
> outer joins? etc etc).
> 2 Cut and paste including the category test and you see all the data that
> would be useful for running tests.
> 3 Finally, run the entire query to ensure the data seen in step two is
> correctly filtered by the final conditions..
> I'm not dogmatic about this, but I find that is a useful habit. A
> compromise when there are loads of columns and you really want very few,
> is to leave them in as comments. On a few occasions I have even found it
> useful to select everything and then wrap that in an outer query that
> targets the one column you want. Something like
> select max(the_date) into my_date from
> (
> select lots of columns used during development
> from tables
> where complicated calculation.
> -- order by the_date
> )
> As a real example, in one application, on various occasions after an
> upgrade the customer questioned the result of a particular complicated
> data calculation. The date query is written in the form shown above. When
> the customer questioned the results it was trivial to run the inner
> portion of the query, with no changes required, and they could see all the
> involved data leading up to the final (maximum) date. That allowed them
> to easily confirm the calculation was indeed correct. I did not structure
> the query that way for them, but it was a useful side effect of my rule of
> thumb from above.
> $0.10

Everyone has an opinion. :)

I can see the validity to yours; I prefer to not include such limiting values in production code once the functionality is verified in dev and test. And that's my opinion. :)

David Fitzjarrell Received on Tue Sep 18 2007 - 08:20:31 CDT

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