Re: anyone who know some good SQL interview questions?

From: Roy Hann <specially_at_processed.almost.meat>
Date: Thu, 9 Nov 2006 07:55:39 -0000
Message-ID: <bv-dnQ1bd_GZQs_YRVnyhQ@pipex.net>

"Pickie" <keith.johnson_at_datacom.co.nz> wrote in message news:1163018184.400606.30640_at_e3g2000cwe.googlegroups.com...
>I had never heard of "aggregate string concatenation", so I googled for
> it. There were only 4 things and in the first I read the comment
>
> <<This also appears to "work", but in fact is officially undocumented
> behavior and thus undefined because there can be no guarantee of the
> order of the rows processed.>>
>
> I was left wondering if knowing something like this was, in fact,
> useful.

If this is the same thing as MySQL's hideous group_concat() aggregate then I think I can provide some more background having recently been challenged to provide a way of doing it in a DBMS that doesn't support such foolishness.

It seems that PHP programmers (at least) are accustomed to use it to concatenate a set of values into a list containing delimiters that can be delivered to their application as a single "row", which they then parse and explode to recover the individual values. These then get put in an array or something.

As far as I can tell they prize it as a performance hack rather than as a logical operation on the data.

Roy Received on Thu Nov 09 2006 - 01:55:39 CST