CODEWITHSUNDEEP
mysqlsqlselectrandomsql-order-by
Th0rndike
Th0rndike

Reputation: 3436

Rewrite a group-by over a randomly-ordered sub-query using only one select

Here's the thing. I'm having 3 tables, and I'm doing this query:

select t.nomefile, t.tipo_nome, t.ordine
from
(select nomefile, tipo_nome, categorie.ordine
from t_gallerie_immagini_new as immagini
join t_gallerie_new as collezioni on collezioni.id=immagini.id_ref
join t_gallerie_tipi as categorie on collezioni.type=categorie.id
order by RAND()
) as t
group by t.tipo_nome
order by t.ordine

It's applied to 3 tables, all in relationship 1-N, which need to be joined and then take 1 random result from each different result in the higher level table. This query works just fine, the problem is that I'm being asked to rewrite this query USING ONLY ONE SELECT. I've come with another way of doing this with only one select, the thing is that according to SQL sintax the GROUP BY must be before the ORDER BY, so it's pointless to order by random when you already have only the first record for each value in the higher level table.

Someone has a clue on how to write this query using only one select?

Upvotes: 2

Views: 224

Answers (2)

Th0rndike
Th0rndike

Reputation: 3436

I was not able to understand the reasons behind the request to rewrite this query, however, i found out that there is a solution which uses the "select" word only once. Here's the query:

SELECT g.type, SUBSTRING_INDEX(GROUP_CONCAT(
i.nomefile ORDER BY
RAND()),',',1) nomefile
FROM t_gallerie_new g JOIN t_gallerie_immagini_new i ON g.id=i.id_ref
GROUP BY g.type;

for anyone interested in this question.

NOTE: The use of GROUP_CONCAT has a couple of downsides: It is not recommended to use this keyword when using medium/large tables since it could increase the server side payload. Also, there is a limit to the size of the string returned by GROUP_CONTACT, by default 1024, so, it's necessary to modify a parameter in the mySql server to be able to receive a bigger string from this instruction.

Upvotes: 1

Andriy M
Andriy M

Reputation: 77687

Generally, if I am not much mistaken, an ORDER BY clause in the subquery of a query like this has to do with a technique that allows you to pull non-GROUP BY columns (in the outer query) according the order specified. And so you may be out of luck here, because that means the subquery is important to this query.

Well, because in this specific case the order chosen is BY RAND() and not by a specific column/set of columns, you may have a very rough equivalent by doing both the joins and the grouping on the same level, like this:

select nomefile, tipo_nome, categorie.ordine
from t_gallerie_immagini_new as immagini
join t_gallerie_new as collezioni on collezioni.id=immagini.id_ref
join t_gallerie_tipi as categorie on collezioni.type=categorie.id
group by tipo_nome
order by categorie.ordine

You must understand, though, why this is not an exact equivalent. The thing is, MySQL does allow you to pull non-GROUP BY columns in a GROUP BY query, but if they are not correlated to the GROUP BY columns, then the values returned would be... no, not random, the term used by the manual is indeterminate. On the other hand, the technique mentioned in the first paragraph takes advantage of the fact that if the row set is ordered explicitly and unambiguously prior to grouping, then the non-GROUP BY column values will always be the same*. So indeterminateness has to do with the fact that "normally" rows are not ordered explicitly before grouping.

Now you can probably see the difference. The original version orders the rows explicitly. Even if it's BY RAND(), it is intentionally so, to ensure (as much as possible) different results in the output most of the times. But the modified version is "robbed" of the explicit ordering, and so you are likely to get identical results for many executions in a row, even if they are kind of "random".

So, in general, I consider your problem unsolvable for the above stated reasons, and if you choose to use something like the suggested modified version, then just be aware that it is likely to behave slightly differently from the original.

* The technique may not be well documented, by the way, and may have been found rather empirically than by following manuals.

Upvotes: 2

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