Re: Little design mistakes that can easily be avoided (1): Concatenated keys and addition of columns

Cimode <cimode_at_hotmail.com> wrote:

>One of the most frequent misconception I face when working is about
>how concatenation became a synonym for addition of attributes.
>Lately, I have had the following example in a board...
>
>COL1 COL2
>ABC DEF
>AB CDEF
>
>It came to my attention lately that several developers make a strong
>confusion onto how unicity should be implemented. (at least the one
>that know that a concatenated keys exists!)...These developers usually
>implement unicity by making a sum between the values in the two
>columns and implementing unicity on that sum.
>
>They end up with a statement such as (assuming we want to insert 'AB'
>in COL1 and 'CDEF' in COL2)

>if not exists(select COL1, COL2 from sometable where COL1+COL2 =
>'ABCDEF')
>begin
>INSERT....
>end
>else
>begin
>REJECT
>end

     This error has a brother who affects numeric columns. (45+79#4579)

[snipped correct way]

>I thought it would be useful to bring that to the attention of this NG
>readers equivalent misconceptions...What about numbering them and
>posting them from time to time...My guess is that a list on *little
>mistakes that can be easily avoided* theme could within a year
>constitute a nice guideline. Any thougths?

     I think it is a good idea.

Sincerely,

Gene Wirchenko

Computerese Irregular Verb Conjugation:

     I have preferences.
     You have biases.
     He/She has prejudices.