ZeldorBlat wrote:
> On May 12, 3:53 pm, "Paul Lautman" <paul.laut...@btinternet.com>
> wrote:
>> ZeldorBlat wrote:
>>> On May 12, 3:44 pm, genzeryk <genze...@gmail.com> wrote:
>>>> Hi Folks!
>>
>>>> I'm a newbie MySQL programmer. While doing my project I have faced
>>>> following problem:
>>
>>>> I have five columns in my table (indexed 'id' and four with
>>>> numerical values). I would like to sum the numerical values into a
>>>> kind of virtual column and sort the table accordingly to the
>>>> decreasing values of the sum (of the virual column).
>>
>>>> I have searched in numerous tutorials and FAQs but have not found
>>>> description of even similar situation.
>>
>>>> Please help
>>
>>>> Best regards,
>>
>>>> Genz
>>
>>> select id, (a + b + c + d) theSum
>>> from someTable
>>> order by (a + b + c + d) desc
>>
>> That seems needlessly wasted on resources, by summing the columns
>> twice. Surely either of these 2 would be better:
>>
>> SELECT
>> `id`,
>> (`a` + `b` + `c` + `d`) `theSum`
>> FROM `someTable`
>> ORDER BY `theSum`
>>
>> or
>>
>> SELECT
>> `id`,
>> (`a` + `b` + `c` + `d`) `theSum`
>> FROM `someTable`
>> ORDER BY 2
>
> Do you know for a fact that MySQL will internally calculate the sums
> twice? If the database engine is properly optimized I would guess
> that it won't.

Yes I do know that for a fact. No matter how well the database engine is
likely to be optimised, it is not going to bother tryjng to recognise that
two distinct pieces of calculation happen to have the same fields. That is
why you are able to refer to previously calculated fields. Optimisers assume
some level of intelligence from the programmer.