Comments on: Unwalking a string with GROUP_CONCAT

By: Checking for string permutation | code.openark.org

Checking for string permutation | code.openark.org — Wed, 24 Feb 2010 21:50:08 +0000

[...] to business: how do we normalize a text using SQL? Well, once again, string walking and string unwalking to the rescue. The trick is to break the string apart (to distinct characters), then re-combine the [...]

By: Rotating SQL graphs horizontally | code.openark.org

Rotating SQL graphs horizontally | code.openark.org — Thu, 23 Jul 2009 13:11:17 +0000

[...] Unwalking a string with GROUP_CONCAT [...]

By: Shantanu Oak

Shantanu Oak — Tue, 14 Jul 2009 14:15:18 +0000

Yes, I got exactly the same warning and after following your tip, I got the results. Thanks.

By: Roland Bouman

Roland Bouman — Tue, 14 Jul 2009 11:30:37 +0000

Hi Shantanu,

do you get any warnings? If group_concat_max_len really is too short to hold the result, you should get a warning:

1 row in set, 1 warning (0.00 sec)

If you want to be sure, don’t set group_concat_max_len to a particular length, just set it to max_allowed_packet. This is the practical maximum of any string length and if you exceed that you will get an entirely different error message, see: http://dev.mysql.com/doc/refman/5.1/en/packet-too-large.html

By: Shantanu Oak

Shantanu Oak — Tue, 14 Jul 2009 08:12:16 +0000

I forgot to mention one thing about group_concat. I am getting 54 strings as against expected 74 when I use group_concat. I do not have access to my.cnf file so I changed the session variable but it did not help.
SET SESSION group_concat_max_len = 1000000;

By: Shantanu Oak

Shantanu Oak — Tue, 14 Jul 2009 06:08:29 +0000

I was just thinking of writing a SP.
Thanks a lot for sharing this.
I selected the int_col because I wanted the exact position of the character.

By: shlomi

shlomi — Wed, 24 Jun 2009 16:59:39 +0000

Hi, Why, with GROUP_CONCAT, of course. You'll need to create the icon (created mine with TheGimp), name it favicon.ico and put it in your theme directory (so one icon per theme) Edit header.php and add the following inside your tag:

That should do the work

By: RT

RT — Wed, 24 Jun 2009 14:46:42 +0000

Hi Shlomi,
Can you please tell how you added that favicon to your blog…

RT.

By: Roland Bouman

Roland Bouman — Thu, 18 Jun 2009 07:26:04 +0000

Hi Shlomi,

Perhaps it was me that wasn’t clear.

In your original post, you mentioned that it was interesting that GROUP_CONCAT does not require a GROUP BY clause. I just pointed out that none of the aggregate functions require a GROUP BY clause, so GROUP_CONCAT is not different from the other aggregate functions in that respect.

I do agree that applying only GROUP BY is aggregation. This brings us to the WTF:

SQL provides two completely different constructs that cause aggregation (as in grouping of rows):
- a GROUP BY clause
- aggregate function

So I’m just saying that IMO, it is strange that the presence of an aggregate function causes aggregation. It would make more sense to me if a grouping construct would first be required before you would be allowed to apply aggregate functions. So in my mind, it would make more sense to write this:

SELECT COUNT(*)
FROM t
GROUP BY NULL

instead of this

SELECT COUNT(*)
FROM t

(of course , GROUP BY NULL is strange, that’s because the keywords were designed to always group according to an expression. That could be solved by making the [BY exprlist] part optional, in which case you would have to write GROUP to convey that you want all rows to be grouped into one row.)

HTH,

Roland

By: shlomi

shlomi — Thu, 18 Jun 2009 06:36:54 +0000

Hi Roland!

Perhaps it’s a matter of semantics, but I’m not sure I understand what you mean.
If I only GROUP BY, is that not considered an aggregation?
For example, in MySQL I can SELECT a non-grouped column with a GROUP BY clause. The selected value is arbitrary, as far as I’m concerned, but can’t it be said that it’s aggregated?

In math we would call this value a ‘representer’ of the group (I’m bending some rules here).